GROWTH STUDIES OF
THE PECAN

S u b m itte d to th e F a c u lty o f th e M ich igan S ta te
C o lle g e o f A g r ic u ltu r e an d A p p lie d S c ie n c e
in p a r tia l f u lf illm e n t o f th e r e q u ir e m e n ts
fo r th e d e g r e e o f D o c to r o f P h ilo so p h y

By
CHARLES L. ISBELL
1928

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SCIENCE LIBRA R>

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TABLE OF CONTENTS
PAGE

IN T R O D U C T I O N A N D S T A T E M E N T O F P R O B L E M

___

3

H I S T O R I C A L _______________________________________________

3

S O U R C E O F M A T E R I A L S __________________________________________

4

P A R T I .— B U D D I F F E R E N T I A T I O N A N D D E V E L O P M E N T ...
M e th o d s _____________________________________
C a tk in f lo w e r b u d d if f e r e n t ia t io n a n d d e v e lo p m e n t _______
P i s t i ll a t e f lo w e r b u d d if f e r e n t ia t io n a n d d e v e lo p m e n t _______
B u d s w ith u n k n o w n f u t u r e d e v e lo p m e n t ______________________

5

S
5
11
15

P A R T I I .— G R O W T H A N D F R U I T I N G H A B I T S ________
19
V a r ie t a l v a r ia t io n in n u m b e r a n d a b s c is s io n o f b u d s _________
19
C h a r a c t e r is t ic s o f p e c a n s h o o t g r o w th ____________________
20
I n f lu e n c e o f k in d o f s h o o t g r o w th m a d e o n e y e a r o n th e
n u m b e r o f f lo w e r s p r o d u c e d a n d n u ts s e t th e f o llo w in g
33
s p r in g ___________________________
I n f lu e n c e o f k in d o f te r m in a l o n p is t illa t e f lo w e r p r o d u c tio n
t h e f o llo w in g s p r in g _____________________________________________ 3 4
F lo w e r p r o d u c tio n fr o m te r m in a l b u d s ______________________ 3 6
I n f lu e n c e o f s e c o n d g r o w th o n n u m b e r o f flo w e r s p r o d u c e d
a n d n u t s s e t t h e f o llo w in g sp r in g ______________________________ 3 6
I n f lu e n c e o f s e c o n d g r o w th o n th e u lt im a t e w e ig h t o f th e n u t 3 8
PART
I I I .— I N F L U E N C E
O F P R U N IN G , D E F O L IA T IO N ,
R IN G IN G , A N D D IS B U D D IN G O N N U M B E R O F S H O O T S
A N D F L O W E R S P R O D U C E D ________________________________
I n f lu e n c e o f p r u n in g o n s h o o t a n d n u t p r o d u c tio n _________
I n f lu e n c e o f d e f o lia t io n a n d r in g in g o n s h o o t a n d fr u it-b u d
f o r m a t io n
_____________________________
I n f lu e n c e o f d is b u d d in g o n s h o o t a n d fr u it-b u d f o r m a t i o n
GENERAL

D IS C U S S IO N

__________________________________________

39
39
43
59
63

S U M M A R Y ______________________________________________

64

ACKNOW LEDGM ENTS

_____________________________________________

66

L IT E R A T U R E

____________________

67

C IT E D

1

,t :' \^\ C r O
Vi

GROWTH STUDIES OF THE PECAN
By C. L. Isbell
INTRODUCTION AND STATEMENT OF PROBLEM
NTEREST IN th e pecan producing industry of the
South has grow n very rap id ly during the last quartercentury. In 1899 th e num ber of bearing pecan trees,
including seedling and budded or g rafted , was given as
643,292. Figures indicating th e num ber of trees of non­
bearing age a t th a t time are not available. These num ­
bers increased to 1,619,521 for bearing and 1,685,066
fo r non-bearing trees by 1909; to 2,672,191 and 2,257,288
by 1919; and to 4,618,297 and 5,120,016 by 1924, res­
pectively. The production of nuts in pounds increased
from 3,206,850 in 1899 to 9,890,769 in 1909 and to 31,808,649 in 1919 ; and th e value from $971,596.00 for 1909
to $7,792,866.00 for 1919 (2).
These g re a t increases have created a dem and among
grow ers and prospective grow ers for inform ation on all
phases of pecan growing. The investigation herein re ­
ported was started to study the grow th habits of the
pecan. Three, more or less sep arate phases of growth,
were studied,— namely, bud differentiation and develop­
m ent; grow th and fruiting h ab its; and, influence of
pruning, defoliating, ringing, and disbudding on the num ­
ber of shoots and flowers produced.

I

HISTORICAL
^ IT T T H IN THE LAST fifty years m any contributions
have been m ade to our know ledge of the time and
n atu re of bud form ation in deciduous fruits and the grow ­
ing habits associated with this function of trees. The
literatu re on this subject indicates th a t in general for
each kind of fru it th e re is a fairly definite period when
fru it bud differentiation ta k es place, and th a t th e initia­
tion of the process depends on th e existence of certain
nutritive conditions within the tissues a t or n ea r th e p a r­
ticular points and time in question.
M oderately vigorous vegetative grow th in deciduous
f ru it trees is essential for m axim um fruit-bud differentition and m axim um fruitfulness. In some instances
3

pruning, defoliating, ringing, fertilizing and other t r e a t ­
ments have exerted little or no influence on th e num ber
of fru it buds f o r m e d ; in others th ey hav e resu lted in in­
creased num bers and in still others in deceased num bers.
W ellington (15) and W iggans (16) hav e given ra th e r
complete sum m aries an d bibliographies of experim ental
w ork bearing on fruit-bud form ation. F o r th a t reason in
this publication reference to oth er literatu re will be m ade
only w here it seems to have some definite bearin g on the
d a ta being rep orted, and th e n only in connection w ith the
specific topic u nder consideration. W h en this investiga­
tion was sta rte d in June, 1922, a p p a re n tly no study of
sim ilar n atu re h a d been m ad e on th e pecan or any other
n u t bearing tree w ith sim ilar g row th and fruiting habits.
SOURCE OF MATERIALS
TV/fTOST OF th e m aterials used in this investigation w ere
obtained from a variety p lantin g of pecans set in
1914 an d from a seedling tree p la n ted about 1900*. The
trees of th e variety planting w ere set 40 fe e t a p a r t each
w ay and peach trees w ere used as fillers until th e first
y ear th e experim ent started. T he seedling tree is located
on the college campus. The soil in w hich th e trees are
growing is sandy, und erlaid w ith clay, an d its n atu ra l
fertility is below th a t req u ired fo r best gro w th of the
pecan. D uring th e experim ent th e trees g rew u nd er
lawn-sod mulch consisting of B erm u da and lespedeza
during each sum m er and hairy vetch and bu r clover d u r­
ing each fall and w inter. F rom year to y ea r th e young
trees received sufficient com plete fertilizer to m aintain
vigorous grow th. These applications w ere m ad e in th e
spring ju st about th e tim e th e nuts w ere a p p a re n tly set.
It was found early in th e experim ent t h a t if th e shoots
of most varieties fail to produce pistillate blossoms they
g enerally abscise th e term inal bud and subsequent grow th
is m ade from lateral buds n e a r th e apical end of th e
shoot. It seemed advisable, th erefore, to m ak e a special
study of these subterm inal buds— th a t is, those axillary
or extra-axillary buds ju st below nuts or below th e point
w here a term inal bud or a term in al p a r t of th e shoot h a d
abscised.**
* T h e se e d lin g prod u ces a good n u t and w ould com e in th e ea rly b loom in g
group a cco rd in g to S tu c k e y ’s (1 3 ) c la s sific a tio n . T h is tr e e h a s b een g iv e n th e
v a riety nam e E arl, fo r P ro f. E arl who p la n ted it. It is referred to in th is paper
under th a t nam e.
** T h e la tera l bud referred to here is u su a lly th e u p p erm o st o f th e su b te r m i­
nal node rem a in in g a fte r th e term in a l bud or th e term in a l p a rt o f th e sh o o t h as
a b scised . On som e v a r ie tie s it is an a x illa ry b u d ; on o th ers it is an e x tr a -a x illa r y .

4

PART I.— BUD DIFFERENTIATION AND
DEVELOPMENT
Methods.— Shoots from which buds w ere taken, as well
as those labeled for study, w ere distributed over the en­
tire tree.
The first sam ples of buds ta k e n for microscopic ex­
am ination w ere killed in Gilson’s killing solution and
in filtrated with paraffin, as outlined by Cham berlain
(3 ). The n atu re of the bud scales and th e close folding
of th e young leaves prevented thorough infiltration, ex­
cept in buds th a t w ere in very active growth, and the
m aterial broke in sectioning. More satisfactory results
w ere secured by removing the bud scales, aspirating for
one hour and th en infiltrating with celloidin. In subse­
quent collections th e scales w ere removed imm ediately
and chrom eacetic acid was used for killing. Sections were
cut to a thickness of about th irty microns, stained with
D elafield’s haem atoxylin, destained with acid alcohol,
w ashed, d eh y d rated with alcohol, cleared with xylol and
m ounted in balsam. Clove oil was used for clearing a
few sections. Double staining with eosin and h ae m a­
toxylin was used with a few sections.
Catkin Flower Bud Differentiation and Development
n r HE PECAN differs from m any other monoecious
^ plants in th a t the stam inate catkin buds and the veg­
etative growing point which la ter may differentiate the
pistillate flow er buds are each enclosed in a sep arate bud
scale or scales, w ithin a common outer scale covering.
W hen the rud im en tary bud form ed in th e axil of the
le af (either before or a fte r th e leaf unfolds) starts rap id
developm ent it form s a mixed bud consisting of three
or more buds under a common bud scale with each bud
enclosed in a se p arate scale. All of these except the
m iddle bud are destined to give rise to catkin buds.
Figures 1 to 13 inclusive are a rra n g e d to show the time
a t which th e catkin differentiation occurs and the pro­
gressive stages in its development.

5

1.— Stuart— May 8, 1926.
2.— Earl— June 19, 1922.
3.— Earl— Dec. 15, 1922.
4.— Earl— Feb. 15, 1923.
5.— Earl— March 5, 1923.
6.— Earl— Jan. 18, 1923.
7.— Delm as— June 30, 1922

6

LEGENDS
Fig. 1.— Stuart bud taken May 8, 1926, from basal part of shoot
just after pistillate flow ers appeared at the top of the shoot.
On the right and le ft catkin flow er buds forming. In cen­
ter vegetative bud.
Fig. 2.— Earl bud taken June 19, 1922, from a node near the de­
veloping nut. On the le ft is a catkin already well developed.
Fig. 3.— Earl bud taken December 15, 1922, from a node near the
nut scar. Catkin on the left well developed.
Fig. 4,— Earl bud taken February 15, 1923, from a shoot that bore
nuts in 1922. It shows on the le ft the extent of the develop­
m ent of the catkin and its hairy condition.
Fig. 5.— Earl bud taken March 5, 1923, from a shoot that fruited in
1922. It shows on the le ft a catkin rather well developed
not long before the buds would have unfolded in the spring.
Fig. 6.— Earl bud taken January 18, 1923, from a shoot that fruited
in 1922, showing one catkin bud with catkins enclosed and
part of another. The vegetative part of the composite bud
is not shown.
Fig. 7.“ Delmas bud taken June 30, 1922, from a node near where
terminal bud abscised between June 23 and June 30. Catkin
on left and right not far advanced.

7

Ffy/o.

/ &

Fig.
Fig'.
Fig.
Fig.
Fig.
Fig.

—

8 .^ E a r l— July 12, 1922.
9.— Earl— July 4, 1922.
10.— Earl— Feb. 1, 1923.
11.— Sluart— April 3, 1926.
12.— Earl— April 4, 1923.
13.— Schley— June 23, 1922.

F~Cq, /3>8

LEGENDS
Fig.

8.— Earl bud taken July 12, 1922, from a node near the de­
veloping nut. It shows a w ell developed catkin on the right
and a vegetative bud in the center.

Fig.

9.— Earl bud taken July 4, 1922, from a node on the basal
part of a fruiting shoot. It shows the development of the
catkin buds on the right and left, and the vegetative bud in
the center.

Fig. 10.— Earl bud taken Feburay 1, 1923, from the basal part of a
shoot that fruited in 1922. It shows the relative develop­
m ent of catkins and vegetative parts of the composite bud.
Fig. 11.— Stuart bud taken April 3, 1926, from a shoot that fruited
in 1925. Enlarged for comparison o f catkin and vegetative
parts o f strong buds near the terminal part of the shoot,
just as growth was starting and bud scales were being lost
in the spring. Parts o f some of the individual staminate
flow ers were broken o ff in sectioning.
Fig. 12.— Earl bud taken April 4, 1923, from a shoot that fruited
in 1922. Enlarged for comparison of catkin and vegetative
parts of buds near the basal parts of a shoot just as growth
was starting in the spring.
Fig. 13.— Schley bud taken June 23, 1922, from the base of a second
growth shoot, showing a catkin bud being abscised at A.

9

DISCUSSION
ITT WILL BE SEEN from F ig u re 1 t h a t catk in flow er buds
* • begin to form
and stam inate flow ers to d ifferentiate
in th e buds along th e base of th e new shoot soon a fte r
g row th begins in the spring. The ra p id ity w ith w hich the
catkins develop soon a fte r th e y are d iffe re n tia ted and
th e continuation of th e ir developm ent until a sho rt time
before blossoming th e following spring is show n in
Figures 2 to 5 inclusive. M any of th e well developed
mixed buds contain th re e or fo u r catkin buds by th e end
of th e grow ing season. F igure 6 is an illustration of
such a bud showing an entire catkin bud an d a portion
of another.
As th e grow ing season advances buds form ed a t newly
developed nodes on eith er first or secondary shoots dif­
feren tiate catkin buds, as is show n in F ig u re 7. As m ight
be expected, due to th e ir d ifferen tiatio n very early in the
grow ing season, th e catkin buds on th e basal portion of
th e shoot are more developed th a n those in buds to w ard
th e term inal p a r t of the shoot. This difference, how ever,
does not continue th ro u g h o u t th e developm ent of th e
catkin ; in fa c t catkins in buds located n e a r th e term inal
p a r t of the shoot finally develop to a m uch g re a te r size,
as will be seen by contrasting F ig u re 8 w ith 9, 3 w ith 10,
and 11 w ith 12 ta k e n from buds to w a rd th e term in al and
basal p arts of th e shoot respectively. These differences
would a p p e a r g re a te r w ere F igures 9, 10 and 12 not
magnified more th a n 8, 3 and 11 w ith w hich th e y are con­
trasted.
W hen second grow th ta k es place— th a t is, w hen lateral
branches develop from mixed buds of th e c u rren t season
—the embryo catkins th a t w ere located in th e buds are
usually abscised, as shown in F ig ure 13. T hey may,
however, rem ain on th e base of th e new shoot an d pro ­
duce catkins the following spring.
A lthough a t th e end of th e grow ing season catkins in
buds tow ard the base of th e shoot are usually sm aller
th a n those in more term in al buds, th e y are la rg e r in pro­
portion to th e vegetative bud w ith w hich th e y are asso­
ciated. This is clearly shown by com paring th e catkins
with th e vegetative p a rts in F igures 10 an d 11.
W hen grow th starts in th e spring th e vegetative p a rt
of well developed buds located n e a r th e term in al p a r t of
the shoot ap p e ars to develop m ore rap id ly th a n th e c a t­
kins w ith which it is associated. T he m ore basal buds,
however, either rem ain d o rm an t or unfold an d produce
10

catkins and very w eak vegetative grow th, the vegetative
parts usually abscising w hen catkins fall. Such a shoot
is show a t point A in Figure 31.
_From th e foregoing it is shown th a t the catkins are
d ifferen tiated in th e composite or mixed lateral buds of
th e pecan almost as soon as th e buds themselves are
form ed and before th e leaves subtending them have a t­
tain ed full size. This differentiation is more or less of a
continuous process, tak in g place as new shoot grow th
is m ade and new la teral buds are form ed. There is a
second period of catkin differentiation corresponding
w ith the laying down of new buds on the second growth.
Considerable developm ent of these catkins takes place
as th e grow ing season advances and is coincident with
the increase in size of the buds*.
Pistillate Flower Bud Differentiation and Development
A S ALREADY STATED, the vegetative center of the
composite or mixed lateral bud of the pecan remains
vegetative from th e time it is form ed until the beginning
of the grow ing period th e following spring. With the
a p p e aran c e of conditions favorable for grow th, both the
vegetative center of the bud and the rudim entary cat­
kins continue th e ir development, the vegetative center
developing more rapidly. The first stage of its grow th
a p p e ars in longitudinal section as an elongation of th e
central axis and a change in th e shape of its crown from
th a t of a broad to a ra th e r n arrow cone. Immediately,
th e re ap p ears a t or ju st below and to the side of the
grow ing point an enlargem ent which tends to give the
grow ing point a shouldered or twisted appearance.
O thers a p p e a r as grow th advances. These protuberances
m a rk the initiation of individual pistillate flowers which
develop rapidly. W hile pistillate flowers are being dif­
feren tiated on th e term inal p arts of the ru dim en tary
shoot, leaves, nodes and internodes are developing ra p id ­
ly and ru d im en tary buds in the axils of th e leaves are
being differentiated and developed. Figures 14 to 22
inclusive are a rra n g e d to show the ap p earan ce of the te r­
minal growing point a t d ifferen t stages before and during
the process of pistillate flow er differentiation.
* T h is is in lin e w ith p relim in a ry rep orts on th is q u estio n m ade by th e
w riter b efore th e H o rticu ltu ra l S e ctio n of th e S ou th ern A g ricu ltu ra l W orkers in
F eb ru ary, 1923, and at th e annual' m e e tin g s o f G eorgia-F lorid a P ecan G row ers’
and th e N a tio n a l Pecan G rowers A sso c ia tio n s, based on earlier stu d ies.
It is
fu r th e r corrob orated by th e in v e s tig a tio n o f W oodroof ( 1 7 ) .

11

p

j .

tyig. 16

V i g . 17

Fig.
Fig.
Fig.
Fig.

14.— Stuart— Dec.
15.— Stuart— Jan.
16.— Stuart— Feb.
17.— Stuart— Feb.

5,
11,
9,
20,

1925.
1926.
1926.
1926.

FL'S’. 2./..

F u. ^ 0 Fig.
Fig'.
Fig'.
Fig’.
Fig*.

18.— Stuart— April 3,
19.— Stuart— April 10,
20.— Stuart-—April 14,
21.-— Sawyer— April 12,
22.— Sawyer— April 12,

1926.
1926.
1926.
1925.
1925.

13

V

LEGENDS
Fig. 14.— A mixed bud of the Stuart taken Dec. 5, 1925, from a
node near the term inal part of a shoot that fruited in 1925.
This bud shows the early w inter stage of developm ent of
the vegetative part of the bud.
Fig. 15.— Stuart bud taken Jan. 11, 1926, from a node near the
terminal on a shoot that fruited in 1925. This bud shows
the midwinter developm ent of the bud.
Fig. 16.— Stuart bud taken Feb. 9, 1926, from a node near the
term inal on a shoot that fruited in 1925, showing catkin
on the right and elongating crown of the vegetative bud on
the left.
Fig. 17.— Stuart bud taken Feb. 20, 1926, from a node near the
term inal on a shoot that fruited in 1925, showing the v eg e­
tative bud with internodes elongating and crown of the
growing point also becom ing elongated preceding pistillate
flow er differentiation. Catkin buds were broken o ff in sec­
tioning.
Fig. 18.— Stuart bud taken April 3, 1926, from a node near the
terminal on a shoot that fruited in 1925, showing an en­
larged longitudinal view through the growing point just
as it was starting rapid growth.
Fig.

19.— The
term inal o f a developing Stuart shoot taken April 10,
1926, showing a stage of pistillate flow er differentiation
further advanced. The bud scales had been lost and the
young leaves w ere beginning to grow rapidly.

Fig.

20.— The
term inal of a developing Stuart shoot taken April 14,
1926, showing the pistillate flow ers further developed than
in any previous figure. The vegetative shoot has made con­
siderable growth; so have the leaves, but the young leaves
were still folded over the cluster of pistillate flow ers so that
it could not have been seen w ithout sectioning.

Fig. 21.— The term inal of a developing Sawyer shoot taken April
12, 1925, just before the pistillate flow ers w ere large enough
to be seen w ithout m agnification.
Fig. 22.— Terminal of a Sawyer shoot taken April 12, 1925, show­
ing pistillate flow er bud developed to the point w here the
ovule was formed.

14

DISCUSSION
1CIGURES 14 to 17 inclusive show the developm ent of
^ the vegetative p arts of the bud during w inter and
early spring. It will be noted th a t th ere ap pears to be a
slight change in the crown of the bud in th a t it becomes
more pointed as th e time for very active spring grow th
approaches. T here is no evidence, however, in these
figures th a t pistillate flow er differentiation has begun.
Figure 18 shows ra th e r clearly how th e vegetative p a rt
of th e mixed bud in the pecan ap p ears as it changes from
a vegetative to a pistillate flow er structure. The crown
of th e bud first elongates th e n shows a slight p ro tu b er­
ance which is a pistillate flow er primodium. T hat the
young pistillate flow ers d ifferentiate and develop ra th e r
rapidly once they have started is well shown by con­
trastin g the extent of th e differentiation and develop­
m ent of pistillate flow ers in Figures 18, 19, and 20. Al­
th ough Figures 21 and 22 are of a different variety and
rep rese n t conditions existing in the spring of 1925 ra th e r
th a n 1926 they show th a t pistillate flowers are develop­
ed r a th e r rapidly. A partly developed ovule is shown at
“ A ” in Figure 22.
The above studies, which covered a period of five years
and included differen t varieties, indicate th a t in east
central A lab am a pistillate flow er bud differentiation in
the pecan tak es place in early spring ju st as bud scales
are drop p ed and rapid grow th is startin g and then pro­
ceeds r a th e r rapidly.
S h u hart (14), and W oodroof and
W oodroof (18) found pistillate flow er bud d ifferen tia­
tion taking place at about the same stage of spring de­
velopm ent of the tree, as was also suggested by the
w riter (7).
Buds With Unknown Future Development
T N TIIE STUDY of the differentiation and development
of stam inate and pistillate flowers a num ber of mis­
cellaneous observations w ere m ade which are of much
interest. P hotographs, some of which are shown in
Figures 23 to 29 inclusive, w ere m ade to record these
findings.

15

LEGENDS
Fig. 23.— Earl bud taken June 27, 1922, from a node near the nut,
showing a vegetative bud in the center and a catkin bud on
each side. The vegetative part was elongating, the scales
were about to be lost and a lateral shoot would soon have
arisen out of the vegetative end.
Fig. 24.— A mixed bud of the Earl taken Nov. 2, 1922, from near
the term inal on a shoot that fruited in 1921. Outer bud
scales had fallen. This is typical of buds that lose the outer
scales during fall and winter.
Fig. 25.— A mixed bud of the Stuart taken Dec. 12, 1925, from a
node near the term inal part of a shoot that fruited in 1925,
showing the developm ent of the leaves while y et in the bud.
The crown o f the vegetative bud is raised. However, it does
not look exactly like the raised crown o f a bud that will
soon differentiate pistillate flow ers.
Fig. 26.— Stuart bud taken Feb 6, 1925, from a node near the ter­
minal of a shoot that fruited in 1924, showing the v egeta­
tive bud apparently starting spring growth with the inter­
nodes elongated and the term inal parts of the bud growing.
Fig. 27.— Stuart bud taker Feb. 6, 1926, from a node near the
term inal o f a shoot that fruited in 1925, showing the de­
velopm ent of the rudim entary leaves and buds in their axils
before the bud scales had been lost from the main v egeta­
tive bud. Such a bud does not appear as if it would d if­
ferentiate pistillate flow ers. Its growing point is very much
like that in Fig. 29, which is known to be vegetative.
Fig. 28.— A true term inal bud of the Earl taken March 26, 1923,
showing general developm ent of the vegetative bud and on
the right at C an undeveloped catkin. Such a term inal bud
may or may not d ifferen tiate pistillate blossoms. On most
varieties they abscise before growth starts the spring follow ­
ing their form ation or just as rapid spring growth starts.
Fig. 29.— Longitudinal section through the growing end o f a la t­
eral shoot arising from an axillary bud during the summer.

Fig'.
Fig.
Fig.
Fig.
Fig.

17

25.— Stuart— Dec. 12, 1925.
26.— Stuart— Feb. 6, 1925.
27.— Stuart— Feb. 6, 1926.
28.— Earl— March 26, 1923.
29.— June 23, 1922.

DISCUSSION

S OME
duce

AXILLARY buds, usually ju st below nuts, pro­
la teral gro w th th e sum m er im m ediately follow ­
ing th e ir form ation. Such buds w ould produce catkins
and m ight d ifferen tiate pistillate flow ers th e following
spring if they did not produce this vegetative grow th.
Figure 23 illustrates one of these buds coming into
grow th. It will be seen from th e abscission lay er a t
th e base of th e catkin flow ers th a t th e y are about to
abscise. The vegetative p a r t of the bud is very d iffe r­
ent in a p p e aran c e from an axillary bud t h a t differen ti­
ates pistillate flow ers w hen it comes into grow th, as was
illu strated in F igures 18 an d 19. If th e axillary buds
are influenced to produce second or la te ra l gro w th due
to defoliation by storms, caterpillars, d ro u g h t or other
causes, th e catkins are not likely to be abscised,, b u t a p ­
p e a r in th e form of blossoms along w ith th e ap p e a ra n c e
of th e second grow th of the vegetative shoot.
During late summer, fall, winter, an d early spring
some of the larg est and a p p a re n tly best developed axil­
lary buds located to w ard th e term in al p arts of th e shoot
m ay lose th e bud scales. Ju st w h a t will be th e fa te of
such buds is u n ce rtain ; usually some of th em drop ; others
m ay grow. F igure 24 is a good illustration of these buds.
T here is ano th er class of buds, pro bab ly not very
numerous, th a t reac h quite a developm ent by early w in­
ter. These buds are interesting because of th e a p p a re n t
developm ent of the crown of th e bud as if pistillate flow ­
ers m ight differentiate. This class is rep rese n ted in
Figure 25.
W oodroof (17) believes such buds to be w in ter-rest­
ing buds, while S h u h art (14) classifies them as pistillate
buds in w inter stage.
T here are also two other types of strong buds th a t are
interesting in th e ir w inter stage, b u t th e ir fu tu re is also
uncertain. They are shown in Figures 26 and 27— th e
form er with internodes a p p a re n tly elongated and th e
grow ing point, ju d g in g from th e d a rk stain it takes, some­
w h at active; th e la tte r with axillary buds well form ed
and w ith a som ew hat unusual ty p e of grow ing point.
It would be interesting to know w h e th e r or not such
rud im en tary axillary buds contain p rim o rd ia fo r catk in
flowers fo urteen m onths before th e y ap p e ar, b u t th e his­
tological technique used in this investigation did not m ake
possible its determ ination.
18

As has been stated, most varieties of the pecan abscise
th e term inal bud. It will be shown la ter th a t term inal
buds th a t do not abscise are not likely to develop catkin
flowers. Figure 28 shows ru d im entary catkins th a t will
probably abscise and a term inal bud th a t m ay or may not
d ifferentiate pistillate flowers.
T h a t th e end of a growing shoot as seen in longitudinal
section is in a p p e aran c e much like th a t of a tru e term inal
just before grow th is resum ed in the spring is evident
from a comparison of Figures 28 and 29.
PART II.— GROWTH AND FRUITING HABITS
Varietal Variation in Number and Abscission of Buds
A/TOST VARIETIES tend to form several buds a t a node,
^
th e num ber depending som ew hat on the vigor of the
shoot and th e location of the node. The S tuart variety
sometimes form s as m any as six buds at a single node. The
size of the buds a t a node usually decreases from the up­
perm ost to th e basal one. In general the buds are suc­
cessively larg er from the basal to the distal end of the
shoot, as shown in Fig 30 A and A', B and B'. W hen ex­
ceptions occur they are generally found w here a vege­
tative shoot slowed down in growth, produced several
short internodes, and grew more rapidly la ter th a t sea­
son. A te m p o ra ry exception may occur a t nodes located
ab ou t th e middle of rapidly-growing-vegetative shoots.
In the former, buds located at nodes in short internode
areas are small, as shown a t D, E. and F in Fig 38. In
th e latter, buds a t nodes n ear the middle of the shoot are
largest.
O ther varieties, of which the Success is an example,
tend to form few buds at each node and a p p e a r to lose
m any of the upperm ost buds of the nodal group by a b ­
scission. The distal bud at every node may drop from
some shoots, the dropping taking place in late summer,
fall, or winter. T here seems to be a tendency, however,
for th e buds a t the subterm inal nodes to be retained for
grow th th e following spring. W hen th e upperm ost bud
a t a node abscises, the second bud usually increased in
size and functions in its place.

19

Characteristics of Pecan Shoot Growth
A FIELD STUDY was m ade of th e grow ing habits of
^ th e pecan from th e tim e grow th sta rte d in spring
until it sta rte d th e following spring. These field obser­
vations show ed t h a t th e pecan m ay— and usually does—
have a g re a t num ber of d iffe re n t kinds of shoots. Some
of th e most common and most im p o rta n t of these types
a re : long and short shoots t h a t fru ited th e y e a r of th e ir
fo rm ation ; long and short shoots t h a t rem ain ed veg eta­
tive b ut d ro pp ed th e term in al bud before th e w in ter fol­
lowing th e ir fo rm atio n ; and long and sho rt shoots th a t
rem ained vegetative and reta in ed th e ir term inal buds the
y ea r of th e ir form ation. All of these types are of much
interest because of th e ir com parative fruitfulness th e
year following th e ir form ation, and because a careful
study of these types and how to cause th e form ation of
a larg e num ber of th e most desirable should give the
pecan grow er b etter returns. These shoots are illus­
tr a te d in F igure 30.
Very short w eak shoots th a t arise out of buds located
to w ard th e m iddle or basal portion of shoots of th e previ­
ous season’s grow th are of interest, because th ey usually
abscise when catkins fall w ith ou t m aking m uch veg eta­
tive grow th or developing pistillate flowers. Such a
shoot is illustrated a t A in F igure 31.
W eak shoots th a t abscise th e term in al or grow ing p a r t
of th e shoot, including undeveloped leaves, ju st about
th e time catkins are falling, and m ore vigorous shoots
showing pistillate flowers, are interesting because they
usually go th ro u g h th e rem a in d e r of th e cu rren t grow ­
ing season w ithout producing f u rth e r leaves or linear
grow th. This ty p e of shoot is illu strated a t A in F igure
32.
Shoots th a t reach m edium length or above and are
vigorous but fail to produce pistillate flowers, as do other
shoots of similar length and a p p a re n t vigor, a ttra c t th e
pecan g ro w e r’s attention because of th e ir failu re to pro­
duce nuts. Such a shoot is shown in F igure 31 a t B.
Shoots th a t reach m edium length or above, produce
pistillate flowers, an d set nuts w hile sh o rt shoots and
shoots of sim ilar length and a p p a re n t vigor rem ain vege­
tative are also of special interest. Such a shoot is illus­
tra te d a t B in F igure 32.

20

W e ak shoots th a t differentiate pistillate flowers which
abscise ju st before or ju st a fte r reaching sufficient de­
velopm ent to be seen w ithout microscopic examination
are of special interest to pecan grow ers because they
rep rese n t one of th e critical steps between large and
small yields. It is reasonable to believe th a t orchard
m an ag em en t practices could be modified so th a t such
pistillate blossoms would set and produce nuts. Shoots
th a t behave this w ay are illustrated in Figure 33.
As has been stated, most shoots of most varieties of
pecans drop the term inal bud and m ake fu rth e r develop­
m ent out of other buds; but, as m any term inal buds are
reta in ed on some varieties and give rise to nut-produc­
ing shoots, such shoots are of im portance. Figures 34 to 36
inclusive illustrate th ree ways in which a shoot may
dispose of its term inal bud.
Shoots th a t develop a com paratively small num ber of
strong buds n ea r th e term inal are usually light bearers.
Even when nuts are produced, the num ber in the cluster
is likely to be small. This type of shoot is illustrated at
A in Figure 37.
Shoots th a t develop m any strong buds tow ard the
term inal are usually heavy bearers. Such a shoot is
shown at A' in Figure 37.
Shoots th a t produce a second grow th while carrying
nuts are of much interest because of the influence of this
second grow th on the location and num ber of flowers
they produce the following spring. The first and second
grow th in shoots of this type are illustrated at B and C
respectively in Figure 37.
Very long shoots are not likely to fru it th e year follow­
ing th eir development, especially if they m ake any form
of second grow th. T h ere are several different types of
these long shoots which m ake some form of second
grow th. They m ake long vegetative shoots, drop the
term inal end, th en m ake additional grow th th a t season;
or they m ake long vegetative grow th which slows down
due to unfavorable grow ing conditions then m ake addi­
tional vegetative grow th, w ithout dropping the term inal
bud. These shoots are illustrated in Figure 38.

21

Fig. 30.
A and A'.— Long and short shoots that fruited and made no more
linear growth until follow ing spring.
B and B'.— Long and short shoots that were vegetative throughout
the growing season and dropped the term inal bud some tim e before
growth started the follow ing spring.
C and C \— V egetative shoots that retained the term inal buds
until the spring follow ing their form ation.

22

Fig. 31.
A.— Very weak shoot that produced very little vegetative growth.
Such shoots abscise when the catkins fall.
B.— V egetative shoot that was apparently vigorous enough to
produce pistillate flow ers, but failed to do so.

23

Fig. 32.— A t A is shown a weak shoot
that is abscising the term inal or growing
point just about the tim e or a little after
the pistillate flow er cluster begins to ap­
pear on more vigorous shoots like B. V arie­
ties that produce a large per cent o f such
shoots as A are not likely to be heavy
bearers.

24

Fig. 33.
A.— A weak shoot that is abscising the pistillate flower cluster just
before the flowers are large enough to be seen by careful examina­
tion without the aid of m agnification.
B.— A shoot that produced a cluster of pistillate flowers which is
abscising w ithout setting any nuts.

25

Fig. 35
Fig. 34

Fig. 34.— Fruiting branches with
tag attached at point where the
term inal bud abscised. The shoots
from subterm inal buds fruited.
Fig. 35.— Fruiting branches with
tag attached at the point where
term inal bud fruited. The figure
also shows a shoot from a sub­
term inal bud that fruited.
Fig. 36.— The term inal bud
from this 1925 VanDeman shoot
did not abscise, but died and shoots
w ere produced from subterminal
buds in 1926.

Fig. 36

26

£
si
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B.— Shoots of the Stuart that set nuts and later, from June to September, produced additional line­
ar growth (C) out of one or more buds. This is a good illusti’ation of how nut bearing shoots make
a second growth.

9

Fig. 38.
A B C.— Shoots that dropped their term inal buds and later made
second growth out of buds near the terminal.
D E F.'— Shoots that slowed down in growth, form ed several
short internodes with weak buds at these nodes, and later made
further linear growth. These finally term inated with a term inal bud.

28

O INCE THE PECAN produces so m any different kinds
^ of shoots, as illustrated in F igure 30 to 38 inclusive,
it was th o u g h t th a t a study of the relative num ber of each
kind m ight th ro w some light on th e bearing habits of the
d iffe re n t varieties. F urtherm ore, inform ation of this
kind m ight also be valuable in suggesting modifications
of some of th e cultural practices to m eet the special r e ­
quirem ents of p artic u lar varieties. Three trees of each
of several varieties w ere selected for this study, which
was m ade ju st before nuts were harvested in th e fall of
1922. M easurem ents were m ade of the length of the
cu rren t y e a r’s grow th of every shoot on each tree and
a t th e same tim e records w ere m ade of th e way each
shoot te rm in ated and as to w h eth e r or not a second
grow th h a d occurred. Table 1 presents these d ata in
some detail and T able 2 sum m arizes them to show the
num ber of shoots th a t fruited, the num ber of nuts ca r­
ried, th e num ber of shoots th a t dropped nuts, and the
num ber of shoots th a t did not fruit.
DISCUSSION
HTHESE DATA SHOW in general th a t: (1) very short
^ and very long shoots are not fruitful, though some
varieties have the ability to fru it over a g re a te r ran ge of
shoot length th a n others; (2) with each variety there
seems to be an optimum shoot length for fruit p ro d u ctio n ;
(3) com paratively few n ut clusters drop after they are
actually set (a cluster of pistillate flowers th a t abscised
before nuts w ere large enough to be pollinated would
leave the shoot ap p a re n tly as if it had abscised its te r ­
minal bud very early in the season and is included with
such shoots) ; (4) th e m ajority of shoots not fruiting,
abscise their term inal bud before the nuts are ready to
fall.
The well know n high-productivity of th e Delmas va­
riety is probably due in p a rt to its ability to fru it on com­
p aratively short shoots, as well as over a wide rang e of
shoot length, though its ab u n d a n t foliage, vigorous
grow th, and good filling qualities are also factors of im­
portance in this connection.
The figures for P ab st carry a suggestion as to why it
is a little slow to come into bearing. It does not fruit
either on very long shoots or very short shoots, as shown
in Tables 1 and 2. Young trees, if vigorous, usually pro­
duce com paratively long shoots; if weak, very short
29

TABLE

I THE NUMBER

AND

LENGTH

OF SHOOTS

'S ;

.lia ii.

^

M
j v>

ft US
M ii.

U j

Uj ®

<£ o f

PRODUCED

BY DIFFERENT

VARIETIES

AND

THE

WAY

THE SHOOTS

TERMINATED

G R O W TH

s 5 ft r>

TABLE

Ui

' I t THE

b e h a v io r

of

shootj

Of
d if f e r e m t

lengths

on

0!F F £R F N T
v a r ie t ie s

shoots. H eavy production m ust aw a it th e g en e ra l a p ­
p earan ce of shoots of m edium length. The F ro tsch er is
known to be a heavy p rod ucer w here it receives an a b u n d ­
ance of nutrients and moisture, fo r it fru its on shoots
having a wide ran g e in length. Possibly failu re of th e
short shoots to reta in th e nuts is responsible fo r its low
yields u n d er unfavorable m oisture and n u trie n t condi­
tions. P etri. (11) show ed this to be tru e w ith olives,
as w as also suggested by Lewis (9) an d by B rad fo rd (1)
w ith apples.
T he ability of th e S tu art to fru it r a th e r freely on
shoots having a considerable ran g e in length probably
explains w hy it seems a d a p te d to such an extended te rri­
tory an d such a wide ran g e of soil types. It is one facto r
in accounting for its general pop ularity am ong pecan
growers. T he d a ta suggest also w hy th e Schley variety is
not often a heavy bearer. Its m axim um n u t production
is on shoots having a r a th e r n arro w ra n g e in le n g th ; no
g re a t p ercen tage of th e shoots ever re a c h th e minimum
fruiting length for th a t variety.
The fa c t th a t th e Success variety fruits on c o m p a ra ­
tively short shoots, coupled w ith th e fa c t t h a t its foliage
is not very luxuriant, probably explains w hy it requires
an a b u n d a n t n u trien t and moisture supply to fill p ro p ­
erly th e heavy crop of nuts th a t th e trees a tte m p t to
carry.
In general it m ay be said t h a t th e longer shoots carry
more nuts th a n th e sho rter shoots, indicating th a t vigor­
ous grow th is necessary fo r m axim um n u t production.
This suggests to th e pecan grow er th e advisability of
furnishing th e trees w ith th e best possible grow ing con­
ditions. This is tru e especially of those varieties th a t
fru it on short shoots and those varieties th a t have a te n ­
dency to produce a larg e p ercen tag e of shoots below
minimum fruiting length, an d which do not have n a tu r ­
ally very vigorous grow ing habits.

32

Influence of the Kind of Shoot Growth Made One Yeai
on the Number of Flowers Produced and Nuts
Set the Following Spring
HP HE INFLUENCE of the kind of shoot grow th m ade
^ one y ear on th e num ber of flowers produced and nuts
set th e following spring was determ ined by labeling, du r­
ing the fall of 1922, long and short vegetative shoots
th a t did not abscise the term inal bud, vegetative shoots
of sim ilar length and size th a t abscised the term inal bud
and fru itin g shoots of similar length and size and reco rd­
ing the behavior of th e shoots arising from them in the
spring of 1923.
These d a ta are shown in Table 3. The different kinds
of shoots are illustrated in Figure 30 A and A', B and B',
C and C\
Table 3.— Influence of Kind of Shoot Growth Made One
Year on the Number of Flowers Produced and
Nuts Set the Following Spring*.
Delmas variety
Shoot that did not
abscise terminal
bud in 1922
Length of shoot
under over
6 in.
6 in.

Shoot that abscised
terminal bud in
1922

Shoot fruiting
in 1922

Length of shoot
under
over
6 in.
6 in.

Length of shoot
under
over
6 in.
6. in

32.9

9.9

32.8

13.4

15.6

Pistillate
clusters form ed 1.3

1.6

1.4

2.0

1.6

1.7

Pistillate
clusters set

1.1

1.4

1.0

1.7

1.1

1.3

Nuts apparently
set
3.6

6.5

3.0

7.8

4.2

5.9

Catkins

9

Stuart variety
5.2

25.0

8.3

21.7

8.5

14.1

Pistillate
clusters formed 1.3

1.9

1.2

1.8

1.8

1.7

Pistillate
clusters set

0.6

1.6

0.9

1.6

1.0

1.5

Nuts apparently
set
1.5

5.5

3.1

7.16

3.4

5.6

Catkins

* F or each group 21 to 28 sh o o ts w ere recorded.

33

DISCUSSION
DATA IN this ta b le show t h a t in g en eral buds out
T HE
of long shoots produced more catkins an d clusters of
pistillate flow ers an d also set m ore nuts th a n did buds on
short shoots of th e sam e type. This is a t variance w ith
W oodroof’s (17) sta tem en t th a t sh o rt shoots produce as
m any catkins as long.shoots. H arv ey an d M u rn eek (5)
found t h a t in th e app le the leaf a re a influenced th e num ­
b er of fruits p er spur. This m ay also be tru e fo r th e
pecan and thus explains w hy th e p ec an g ro w er who gives
his pecan grove w h a t it requires to m ak e good v e g e ta ­
tive gro w th produces more pecans p e r acre th a n th e
grow er who does not, although each o rch ard m ay have
th e sam e nu m b er of shoots fruiting.
The fa c t th a t short shoots of th e S tu a rt variety th a t
did n o t abscise th e term inal buds th e previous y e a r— as
show n in T able 3— ap p a re n tly set an av erag e of 1.5 nuts,
while shoots of sim ilar length t h a t fru ited or d ro p p e d th e
term inal bud produced 3.4 and 3.1 respectively suggests
th a t short shoots of this typ e are not as likely to fru it as
short shoots of sim ilar length t h a t te rm in a te differently.
This m ay indicate t h a t these shoots continue vegetative
grow th until r a th e r late' in th e season and do not have
enough stored food to initiate pistillate flow er d iffe re n ­
tiation, or, if enough to initiate it, no t enough to c a rry it
to setting. If this is tru e and it could be app lied to such
varieties as th e Schley, which often m akes m any short
shoots th a t do not fruit, it m ay explain w hy th e variety is
not a re g u la r bearer. T he sum m er of 1925 w as so dry
th a t th e re was no second grow th. The Schley variety
fru ited very well in 1926, which suggests f u r th e r th a t
this m ay be true. It is interesting to note in this connec­
tion th a t Roberts (12) found w ith th e plum t h a t blossom
buds form ed earlier on shoots t h a t te rm in a te d gro w th
earlier.
Influence of Kind of Terminal on Pistillate Flower Pro­
duction the Following Spring
n r O OBTAIN D A TA on th e influence of th e kind of
A term inal form ed by a shoot on th e p erfo rm an ce of the
laterals grow ing out from it, a n u m b er of shoots w ere se­
lected a t ran d o m on trees of each of several varieties in
th e fall of 1925 and exam ined th e spring 1926. The shoots
w ere gro u ped to include (1) those t h a t te rm in a te d w ith
34

term in al buds, (2) those th a t abscised term inal buds, and
(3) those th a t bore nuts. The d a ta are sum m arized in
T able 4 w hich follows:
Table 4.— Influence of Kind of Terminal Formed in the
Season of 1925 on the Number of Shoots That Pro­
duced Pistillate Flowers the Spring of 1926.
V ariety
No. shoots
examined
Per cent shoots
t h a t term inated
1925 with term inal
bud
Per cent term inal
buds grow ing and
producing pistillate
flow ers
Per cent terminal
buds not growing
but pistillate flow ­
ers being produced
on growth from lat­
eral buds

Sawyer
(2 trees)

Tesche Centennial Schley VanDeman
(2 trees) (1 tree) (2 trees) (2 trees)

188

116

105

192

203

33.51

20.68

83.80

40.11

9.35

77.77

91.66

68.18

74.02

*0.00

17.46

8.33

1.13

9.09

100.00

20.68

0.00

43.75

25.61

100.00

0.00

84.52

100.00

58.62

16.19

16.14

65.02

0.00

5.88

12.90

0.00

Per c e n t shoots
t h a t term inated
1925 with nuts
16.48
Per cent shoots ter­
minating with nuts
1925 and produc­
ing pistillate flow ­
92.90
ers 1926
Per cent shoots ter­
m inating 1925 with
extra-axillary buds.
producing flow ers
50.00
1926
Per cent shoots ter­
minating 1925 in
extra-axillary buds
and not producing
pistillate flow e r s
6.38
1926

* V anD em an sh o o ts n o t in cluded in th is cou n t w ere found th a t produced
p is tilla te flo w ers ou t of term in a l buds.

It is shown in Table 4 that it is possible for all varieties to fruit
from true term inal buds, but the percentage of terminal buds that
fru it is very low in some varieties. On the other hand, some varie­
ties fru it from a large percentage of the terminal buds formed.

35

F lo w er P rod u ction from T erm in al B uds

N MOST VARIETIES a la rg e p erce n tag e of th e terminal buds are abscised before g ro w th sta rts th e
spring following th e ir form ation. However, a stu d y was
m ade of several individual shoots th a t te rm in a te d with
term inal buds th a t did not abscise to determ ine exactly
how these buds behave th e following spring. T able 5
presents these data.
Table 5.— Behavior During Spring of 1926 of Terminal
Buds Formed in 1925
V ariety

Frotscher
Delmas
Stuart

No. shoots Per cent
Per cent
Per cent Per cent
term inal
term inal
with ter­ of term inal terminal
minal buds buds pro­ buds grow ­ buds fa il­ buds pro­
ducing ing but fa il­
ing to
ducing
ing to
grow
catkins
pistillate
flow ers
produce
pistillate
flow ers
116
233
42

18.10
97.85
97.64

0.00
1.28
0.00

81.90
0.85
2.38

0.25
10.72
0.00

These data show that the term inal bud is unlike m ost m ixed or
extra-axillary buds in the pecan in that it usually does not contain
catkin flow er buds that develop far enough to furnish pollen. On
the other hand, they show that in some years true term inals in some
varieties give rise to a high per cent of pistillate-flow er-bearing
shoots.
Figure 34 illustrates a shoot on which the term inal bud failed to
grow, and fruiting shoots arose from buds where the term inal
abscised. Figure 35 illustrates a shoot on which the term inal bud
produced a fruiting shoot. In this illustration is shown also a fru it­
ing shoot that arose below the term inal fruiting shoot.

Influence of Second Growth on Number of Flowers Pro­
duced and Nuts Set the Following Spring
A FIELD STUDY of th e grow ing habits of the pecan,
Table 1, show ed th a t some varieties m ake a consider­
able am ount of “ second g ro w th ” a fte r nuts are set. T able
6 presents d a ta on how th e following season’s vegetative
grow th and flow er production from these secondaries
com pare w ith th a t of shoots not m aking a second grow th.

36

Table 6.— Influence of Second Growth on Number of
Flowers Produced and Nuts Set the Following Spring*.
Shoots that did
not produce
second growth

Shoots that produced
second growth

Length of growth Length of growth Length of growth
Under Over
Under 6 inches Over 6 inches
6 in.
6 in.
1st
2nd
1st
2nd
growth growth
growth growth
No. catkins

4.3

6.0

3.1

10.0

1.6

11.0

No. pistillate
clusters form ed

2.3

2.1

1.2

2.4

1.2

2.3

No. pistillate
clusters set

1.3

1.1

0.4

1.7

0.13

1.6

No. nuts appar­
ently set

4.8

4.2

0.04

7.0

0.4

6.9

* F or each group 27 to 32 sh o o ts th a t fru ited in 1922 w ere used.

DISCUSSION
These d a ta show :
(1 )
That the production of laterals incident to “second growth”
increases the number o f both catkins and pistillate clusters; and (2)
that the laterals of shoots that make a second growth produce more
catkins and more pistillate clusters than the primary portions o f the
same shoots.
Incidently they substartiate the statem ent made
earlier to the e ffect that there is a long period of catkin bud d if­
ferentiation. Gourley (4 ) has shown that in the Baldwin apple
there is a second period of fru it bud form ation the latter part of
the summer and early fall, as evidenced by fru it bud formation on
the terminus of the second growth. It appears that the catkin flow ­
ers that have already been differentiated when second growth oc­
curs do not have an equal chance for developm ent and flow ering
with those on the secondary shoots.

Field observations indicated th a t long shoots th a t m ade
a “ second g ro w th ” w ithout dropping the term inal bud of
the prim ary shoot, as shown in Figure 38D, E, F, are not
very likely to fru it the following season. Heinicke (6)
believed th a t a fte r grow th finally ceased on long apple
twigs the time rem aining for active assimilation was in­
ad equ ate for a b u n d a n t storage in th e buds. This is prob­
ably true also of those pecan shoots shown in the figure
to which reference has ju st been made. These shoots
in th e pecan are ch aracterized by a group of very short
internodes, a t th e nodes of which are located small w eak
buds th a t w ere subtended by poorly developed leaves.
Heinicke (6) says th a t the exact cause of variation in
bud vigor in th e apple is not known. In the pecan, on
37

shoots as show n in F ig ure 38 E an d F, th e buds located
at nodes not f a r back of the term inal are w ea k because
the shoot was about to te rm in ate g ro w th by abscising
th e term inal p arts w hen additional lin e ar g ro w th w as
m ade. However, th e w eak foliage an d buds on th e p a r t
th a t w as ab ou t to abscise never recovered enough to become vigorous.
Influence of “Second Growth” on the Ultim ate W eight
of the Nut
1CIELD OBSERVATIONS suggested t h a t developing nuts
on shoots th a t m ad e second g row th m igh t be sm aller
th an nuts on shoots m akin g no second grow th. N uts w ere
harvested from fo u r varieties in th e fa ll of 1923, th re e
varieties in 1925, and from one v ariety in 1926, and
w eighed to determ ine w h eth e r these differences actually
existed. Table 7 presents these d ata.
Table 7.— Number of Nuts per Cluster and the Compari
son of the W eight per Nut on Shoots That Did
and Did not Make Second Growth
Shoots that did not
make second growth
No. Av.
trees No.
nuts
per
cluster

Av.
wt.
nuts
per
cluster

Av.
No.
nuts
per
cluster

Av.
wt.
nuts
per
cluster

Av.
wt.
per
nut

gms.
8.00

3.00

gms.
21.47

gms.
7.15

18.08

6.55

2.78

19.10

6.87

2.11

20.34

9.65

2.36

21.08

8.94

2

2.30

14.31

6.22

2.76

15.39

5.54

Tesche

2

3.04

17.63

5.80

3.71

21.02

6.16

1925

Stuart

5

2.25

20.20

9.05

No second growth
occurred

1925

Success

5

1.85

19.15

10.30

No second grow th
occurred

1925

Frotscher

4

2.10

19.32

9.16

No second growth
occurred

Year

V ariety

1923

Stuart

2

2.11

gms.
16.89

1923

Success

1

2.72

1923

Frotscher

2

1923

Tesche

1926

38

Av.
wt.
per
nut

Shoots that made
second growth

DISCUSSION
T T IS EVIDENT th a t in 1923 the average w eight of nuts
p e r cluster and average num ber of nuts p er cluster
were g re a te r in all varieties com pared w here the shoots
carrying nuts m ade second growth, but the average
w eight p e r n u t was sm aller with three of the four varie­
ties com pared. The same was true of the Tesche in
1926, except th e average w eight per nut was g re a te r on
shoots m aking a second growth.
These facts indicate strongly th a t the shoots which for
some reason are carrying the g reatest num ber of nuts
are also most likely to produce a second growth. It is
also evident th a t th e ap p earan ce of second grow th influ­
ences th e w eight of th e m atu re nut. A study of the rain ­
fall a t A uburn during these years indicates th a t the ini­
tiation of second grow th is associated with heavy rainfall
during th e early growing period, and th a t the am ount of
rain fall during late sum m er and early fall influences the
filling and, in turn, the w eight of m ature nuts.
PART III. INFLUENCE OF PRUNING, DEFOLIATION,
RINGING, AND DISBUDDING ON NUMBER OF
SHOOTS AND FLOWERS PRODUCED
Influence of Pruning on Shoot and Nut Production
TO ECAN GROWERS have generally believed th a t w hen
a pecan shoot is pruned (i. e. h ea d ed back) it will
not produce nut-bearing laterals the following season.
Field observations in 1923 indicated th a t this notion does
not accord w ith th e facts (see Fig. 39) and raised the
question as to w h eth e r or not certain types of pruning,—
fo r example, th a t incident to cutting scion wood,— on
some varieties m igh t be practiced w ithout injury and
w ith the possibility of favorably influencing the quality
and q uantity of nuts. Following these observations an
effort was m ade to determ ine how shoots on different
varieties would respond to varying am ounts of heading
back at d ifferen t dates.
Shoots th a t fru ited in 1923 w ithout producing a sec­
ond grow th and shoots th a t fruited and produced a
second grow th w ere p run ed at different times and with
various degrees of severity. Shoots th a t m ade a second
grow th w ere pru ned to determ ine w h eth e r or not th e
secondaries would fru it a fte r pruning and w h eth e r the
first grow th would fru it if all the second grow th were
cut aw ay. T he season of 1924 was not a heavy crop year
39

40

and th e d a ta secured th a t season w ere not very extensive.
They did, however, indicate th a t buds below cuts on all
of th e th re e classes of shoots mentioned could be made
to yield fruit-bearing laterals if the shoot would have
norm ally fruited out of buds n ear th e term inal w ithout
pruning.
In ano th er experim ent, shoots th a t were vegetative
th ro u g h o u t 1923 were pruned so th a t th e re would be
four, eight, twelve, or sixteen nodes left a fte r pruning.
These were ta g g ed and their grow th com pared with th a t
from checks (i. e. unpruned shoots) in the spring of
1924. The resulting records are presented in Table 8.

Fig. 40.— Nut-bearing clusters of the Success.
A.— Pruned shoot with two buds arising at the same node and
fruiting.
B.— Pruned shoot fruiting out o f a bud far below where shoot
was headed back.
C .— Shoots arising below pruning cut, fruiting and making second
growth. All cuts made at point indicated by arrow.
41

Table 8.— Influence of D ifferent D egrees of Pruning on
Production of Nuts the Follow ing Year
No. nodes
left after
pruning
4

8

V arieties
Delmas Frots- Pabst Schley Stuart Suecher
cess
13

No. shoots pruned

9

5

10

2

12

Shoots produced
per pruned shoot

2.53

2.11

2

1.7

2.5

1.66

N uts produced per
pruned shoot

0.0

0.0

0.0

0.0

0.0

1.58

7

8

6

15

No. shoots pruned

11

16.0

s

12

16

Shoots produced
per pruned shoot

3.2

2.81

2.28

2.5

2.5

2.25

N uts produced per
pruned shoot

0.33

0.0

0.0

1.0

0.33

1.37

9.0

7.0

8.0

8.0

10.0

No. shoots pruned

21.0

Shoots produced
per pruned shoot

4.61

3.55

2.71

2.75

4.62

2.8

N uts produced per
pruned shoot

1.61

0.0

0.32

0.87

2.62

1.7

3.0

3.0

1.0

3.0

No. shoots pruned

23.0

12.0

Shoots produced
per pruned shoot

5.65

4.66

3.0

3.0

5.0

2.66

N uts produced per
pruned shoot

0.0

1.0

0.3

5.0

3.33

1.58

20.0

17.0

11.0

7.0

. 3.65

3.0

3.09

3.14

4.3

2.09

0.0

0.18

0.71

1.76

1.18

Check
No. shoots not
Not
pruned pruned
Shoots produced
per shoot
Nuts produced per
shoot

0.0

26.0

22.0

These data, though not extensive, indicate that some varieties may
respond very satisfactorily to certain amounts of heading back. It
was observed that, at least in the Success variety, it is possible for
two of the buds at a single node to produce fru itin g laterals and
that, even though the buds near the cut on a pruned shoot m ay n ot
grow, those lower down m ay grow and fruit, and that pruned shoots
may produce laterals that fru it and make second growth. This is
shown in Figure 40, A, B and C respectively.

42

DISCUSSION
13 ESULTS OBTAINED from heading back shoots as
shown in F igure 40 A explain why shoots on some va­
rieties m ay drop the extra-axillary bud a t practically
every node and th en fru it freely on shoots developing
from buds th a t are left.
P a rtrid g e (10) found th a t the fruiting capacities of
g rape buds vary with their position on the cane. There
is a suggestion th a t the same may be tru e to a certain de­
gree w ith referen ce to the position of the bud on the
shoot of certain varieties of th e pecan. Not only does the
position of th e bud on the shoot influence its ability to
d ifferentiate a pistillate flow er cluster and its likelihood
of fruiting, b ut th e position likewise influences its fru it­
ing capacity,— i. e . , the num ber of flowers th a t th e clus­
te r produces.
A p paren tly th e pecan shoot may be com parable in
some degree with th e rasp b e rry and fig in th a t the buds
a t th e most basal nodes are potentially fruit buds, though
they are usually not utilized for fruit production unless
the more term inal p arts are removed by pruning, or other­
wise.
Influence of Defoliation and Ringing on Shoot and Fruit
Bud Formation
A YOUNG V anD em an pecan trees which h ad been killed back and sprouted out n ea r the point of union of
stock and scion was th o u g h t to be from the stock. In
Ju ly 1921 an a tte m p t was m ade to top w ork this tree by
means of ring budding one of its branches about eight
inches from th e m ain tru nk . The ring or patch of
b ark carrying th e bud lived but the bud which it carried
did not grow and the branch was not cut back. E xam ina­
tion of the tree on October 16, 1922, led to the discovery
th a t it carried five nuts of the V anD em an variety, show­
ing th a t it h a d not been killed back to the stock and th a t
the sprout h ad sprung from n ea r the base of the scion.
T hree of th e five nuts w ere on the branch th a t h ad been
girdled incident to th e budding operation, suggesting
th a t ringing in this case m ight have prom oted ra th e r
th a n interfered with fru it bud formation.
Following these observations experim ents w ere plan­
ned to determ ine the influence of defoliation, ringing and
certain combination trea tm en ts on fruitfulness in the
43

pecan. Shoots on Delmas an d P a b st tree s w ere tr e a te d
as follows during th e weeks of Ju ly 13-26, 1924:
(1 ) V egetative shoots 5 to 10 inches long w ere partly d efo li­
ated by severing the rachis (extension of the petiole) beyond
the two basal leaflets. A ny late summer grow th produced by
these shoots was le ft undefoliated.
(2 ) V egetative shoots w ere defoliated as in (1) and the new
growth that developed was prom ptly defoliated in a similar
manner. As a m atter of fact, such a sm all quantity appeared
that the records of these shoots were grouped w ith those of
(1) when fin al records w ere made.
(3 ) On vegetative shoots as in (1) a ring of bark one-fourth
inch wide, located four nodes from the base, was rem oved and
le ft unwrapped.
(4 ) From another group of shoots sim ilar to those used in
(1 ) the ringed portion was im m ediately wrapped with waxed
cheesecloth, such as is used in budding the pecan.
(5 ) Other vegetative shoots similar to (1) w ere ringed as in
(3) and partly defoliated as in (1).
(6) Still other vegetative shoots w ere ringed as in (3 ) and
partially defoliated to the base of the 1924 growth.
(7) V egetative shoots of medium vigor, 5 to 10 inches long
and distributed throughout the tree were selected, le ft untreated
and labeled as checks.

n p H E S E SHOOTS w ere exam ined th e la tte r p a r t of Aug^ ust. On th e Delm as w here th e w ra p was wide enough
to cover th e ringed p a r t and re st on th e b a rk on either
side, th e ringed a re a h a d nearly or entirely h ealed . In
m any cases th e callus or new b ark form ed a t rig h t angles
to th e b ran ch was sufficient to force its w ay th ro u g h
two layers of w rap p in g cloth. T h ere w as a lim ited
am ount of new term inal grow th, p e rh a p s to an ex ten t of
about eight leaves. W here th e shoots w ere defoliated
and the w ra p p ers w ere so n arro w as to fit into th e ringed
spaces preventing th e callus from bridging th e wound,
th e bud ju st below th e ring developed into a sh o rt shoot
in a num ber of instances.
P a b st shoots h a d callused about th e sam e as those on
th e Delmas. However, th e re was no ad dition al grow th
from th e term inal bud. In only a few instances did la t­
eral buds even on th e most vigorous shoots give rise to
a second grow th. In both varieties th e re was a tend en cy
for p a rtly defoliated shoots to drop even w h a t foliage
was left. This was most m a rk e d to w ard s th e base of th e
shoot an d on th e less vigorous trees.
By S eptem ber 23, m any of th e tr e a te d shoots oh P a b st
variety w ere dropping the foliage. In some cases this
condition extended only to th e ring an d in oth ers to th e
base of th e shoot. Similar shoots on Delmas h a d less te n ­
dency in this direction.
44

The tre a te d shoots w ere examined th e spring follow­
ing tre a tm e n t to determ ine the influence which different
trea tm en ts an d d iffe re n t degrees of healing of th e ringed
portion h a d on location and num ber of catkin flowers,
pistillate flowers, and vegetative shoots th a t appeared.
A very few of th e shoots with rings left unw rapped
died before th e spring following treatm en t. Figure 41 il­
lustrates how this class of shoots a p p e ared the following
spring. It will be noted in this illustration th a t the m a­
jority of th e vegetative response is out of the bud lo­
cated ju st below th e ringed a re a as indicated by the a r ­
row. It will also be noted th a t th ere was a ra th e r heavy
production of catkin flowers at the first nodes below the
ring.
Delmas shoots on which the ring failed to callus— due
to in terference of th e w ra p — produced vegetative growth
and catkin flow ers im m ediately below th e ringed area
and a rosette of several poorly developed leaves at the
term inal th e spring following treatm en t. Figure 42 il­
lustrates this type of shoot.
P ab st shoots on which the ring failed to callus due to
interference of th e w rap responded the following spring
in two more or less differen t ways. One was with vegeta­
tive grow th and catkin developm ent below the ring very
similar to such shoots on Delmas. These varieties how ­
ever showed g re a t contrast in th e response secured on the
term inal portion of th e tre a te d shoot. The P ab st pro­
duced m any catkins th a t died before reaching more th a n
h alf norm al development. This response is well illus­
tra te d in F ig ure '43.
O th e r P ab st shoots on which the callus failed to cover
the ring gave vegetative grow th, pistillate and catkin
flow er response below th e ringed area and a term inal
catkin response th a t was so w eak as to almost fail to
throw off the bud scales before dying. Figure 44 illus­
trates a shoot th a t gave these responses.
The differences pointed out above betw een the two
varieties m ay be a t least p artly due to difference in rela­
tive m aturity of Delmas and P ab st shoots at time of
ringing. W ith shoots of both Delmas and P ab st varieties
on which th e ringed a re a partly callused, the vegetative
response and catkin flow er develpom ent tended to be
distribtued over th e entire shoot w ith the g reatest re­
sponse occurring ju st below the ring and at th e term inal
part, while th e w eak est vegetative and catkin response
seemed to a p p e a r not f a r above the ringed area.
These responses fo r Delmas and P ab st shoots are II45

lustrated by F igures 45 an d 46 respectively. W ith shoots
of Delmas and P a b st on w hich th e rin ged a r e a alm ost or
entirely callused th e catkin flow ers a p p e a re d ju st below
th e ring and to w a rd th e term inal, w ith catkins absent
ju st above th e ring. The flow er an d vegetative response
of this class of Delmas shoots is illustrated in F ig u re 47,
while th a t of th e P a b st is illu strated in F igures 48 and 49.
In connection w ith these observations on th e influence
of ringing, F igure 50 is included to illu strate a fruiting
shoot arising out of an adventitious or reserve bud two
years a fte r being ringed by a wire. The influence of de­
foliation on th e production of flow ers is illu strated in
Figure 51.
In vigorous apple and p ea r trees, girdling an d ringing
have frequ ently induced fruitfulness in th e portion above
the g i r d l e ; in th e pecan like results have not been se­
cured. However, it m ust be recognized th a t in th e apple
th e tim e th e operation is p erform ed m ak es a g re a t dif­
ference in th e results, as has been found to be tru e in case
of the pecan (8 ). F u rth erm o re, girdling th e ap ple is
g enerally practiced on wood distinctly older th a n th a t
used in this w ork on th e pecan. W ith all these allow ­
ances m ade, however, th e effects actually a p p e a rin g are
certainly quite d iffe re n t from any th a t could be expected
in the apple, and suggest th e possibility of a differen t
chemical basis fo r blossom differentiation. This w ould
not be surprising in view of th e d ifferen t periods of dif­
ferentiation in the ap p le and th e pecan.

46

«l-'W*‘

Fig. 41.— A Delmas shoot where the ring was not wrapped.
The part of the ringed shoot above the ring died before
growth started the follow ing spring.

47

Fig. 42.— A Delmas shoot on which the ring did not callus. V ege­
tative growth and catkins w ere produced im m ediately below the
ring, and a whorl of small leaves on the term inal part of the
ringed shoot the spring follow ing ringing in summer.

48

Fig. 43.— A Pabst shoot on which the ring did not callus.
It produced vegetative growth and catkins below the ring,
and a few weak catkins toward the term inal part of the
ringed shoot the spring follow ing ringing.

49

Fig. 44.— A Pabst shoot on which the ring did not callus. It pro­
duced vegetative growth, catkin flow ers and a cluster of pistillate
flow ers below the ring, and weak catkin flow ers near the term inal
part of the ringed shoot the spring follow ing ringing.

50

Fig. 45.— A
healed partly.
appeared above
low ing ringing,

Delmas shoot on which the ring
The vegetative growth and catkins
and below the ring the spring fo l­
and both were vigorous.

51

Fig. 46.— A Pabst shoot on which the
ring healed slightly. The vegetative growth
and catkins appeared the spring follow ing
ringing above and below ring.

52

Fig. 47.— A Delmas shoot on which the wound caused by ringing
almost healed. The vegetative growth and catkin flowers appeared
above and below the ring and pistillate flow ers only above the ring.

53

48
Fig. 48.—
A Pabst shoot on which the ring healed entirely. The catkin flow ers appeared below
and above the ring while the vegetative growth
and pistillate clusters appeared only above the
ring. Foliage removed to show pistillate flowehs
on young shoots.

Fig'. 49.— A Pabst shoot where the ring partly
healed. N uts set both above and below the ring.
Those below the ring matured, while those above
dropped before m aturity.

55

Fig. 50.— A shoot arising and fru iting out of an
adventitious or reserve bud two years after being
ringed by a wire. The variety is Sawyer and the
tree is about eight years old.

56

Fig. 51.— Shoots that were (A ) and were not (B ) defoliated.
Otherwise these shoots were apparently alike. They were on the
same branch. N ote that B produced more catkins than A and it
also produced a cluster of pistillate flow ers while A did not.

B E T W E E N MAY 1 and May 15, 1925, the trea ted
shoots w ere exam ined fu rth e r to determ ine the num ­
ber of branches th a t had arisen both above and below the
rin g ; also the vigor of these new shoots and the num ber
of pistillate clusters and stam inate flowers produced. A
young shoot th a t m ade little grow th and carried small,
light colored leaves was term ed very w ea k ; one th a t
m ade a long, well developed grow th and carried large,
deep green leaves similar to the best arising from buds
on the u n treated shoots was term ed very vigorous. The
terms weak, medium, and vigorous rep resen t gradations
between these two extremes.
Figures indicating relative vigor of the tre a te d shoots
as reflected by vigor of young shoots arising from them
were obtained by assigning to very vigorous, vigorous,
medium vigorous, w eak and very w eak shoots values
of 10, 8, 6, 4 and 2 respectively and m ultiplying the num ­
ber of shoots occurring in each group by the assigned
value and ad din g to secure total vigor, th en dividing by
num ber of shoots tre a te d to get averages. By this method
of estim ating vigor of th e tre a te d shoots m ay be as low as
zero or g re a te r th a n ten.
The records th a t w ere obtained are shown in Table 9.
57

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Table 9.— Influence of Defoliation, Ringing, Ringing and Defoliation during summer of 1924
Shoot Growth and Fruit-Bud Formation— 4 to 6 trees were used in all cases and
from 5 to 77 shoots were treated

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I T IS OBVIOUS th a t d a ta in T able 9 w ere obtained from
A a com paratively small num ber of shoots but th ere seems
to be sufficient evidence to show th a t:
(1 ) The number of young shoots produced above the ring,
where it was partly or entirely healed, was in all cases greater
than the number below the ring.
(2) The number o f young shoots produced above the ring,
when no part of the ring healed, was less in all cases than the
number produced below the ring.
(3) The vigor of the shoots produced above and below the
ring depended on the exten t to which the ring had healed.
Where it had healed the vigor of shoots was greater above the
ring than where it did not heal.
(4) D efoliation tended to increase the number of shoots
produced and lower the vigor of shoots produced in both varie­
ties.
(5 ) D efoliation m aterially cut down the number of clusters
of pistillate flow ers in both varieties.
(6) D efoliation cut down catkin flow er production in the
Delmas and apparently increased it in the Pabst variety. The
increase in case of the Pabst may have been due to possible
error in estim ating and figuring catkins, since it is possible
that only the weak shoots o f the check held catkins after the
storm.
(7 ) Ringing and preventing the ring from healing reduced
catkin and pistillate cluster form ation in both varieties more
than did defoliation.
(8) Ringing and allowing the ring partly or entirely to heal
resulted in a slight increase in the number of catkins in Pabst
and Delmas and decreased the number of pistillate clusters in
both varieties. The decrease was considerable in case of Delmas.
(9) Ringing and defoliating to the base of the shoot and then
not allowing the ring to heal resulted in reduction of catkins
and in the com plete prevention of nut cluster form ation in
Delmas and in reduction in case o f Pabst.
(1 0 ) Ringing and defoliating to the base of the shoot and
then allowing the ring to partly or entirely heal resulted in
an increase in number of catkins and pistillate clusters in
Pabst, and to a considerable reduction in catkins and prevention
of pistillate cluster form ation in case of Delmas.
(11) In general preventing the healing of the ring reduces
flow er production.

Influence of Disbudding on Shoot and Fruit-Bud Forma­
tion
H rO OBTAIN definite d a ta on the degree of uniformity
or similarity betw een th e several buds at a single node,
as m easured by the kind of shoots to which they give rise,
a series of disbudding experim ents was sta rted on Stuart
and P ab st trees.
Shoots th a t carried nuts in 1925 were labeled and the
first, first and se c o n d ; first, second and th ird buds at
the first six nodes below th e n u t scar w ere removed on
d ifferent trees a t w eekly intervals from March 24 to
April 14, 1926, and th e ir subsequent records checked
against those of sim ilar shoots from which no buds were
removed. T he response to disbudding for the two varie­
ties was very similar. D ata fo r S tu art are given in sum­
m arized form in T able 10 and illustrated in Figure 52.
59

Stuart)

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35

(variety
Flowers
and
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on Production
of Disbudding
10.— Influence

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61

DISCUSSION
T T WILL BE seen from T able 10 th a t norm ally most of
th e vegetative gro w th and pistillate-flow er-producing
shoots arise from buds located a t nodes n e a r th e term inal
p a r t of th e shoot. Similarly th e m ajo rity of th e catkins
are norm ally developed from buds located a t nodes n e a r
th e term inal p a r t of th e shoot. However, some of th e
catkins are norm ally produced from buds extending al­
most to th e base of th e shoot.
The second bud, as well as th e first, a t a given node
m ay produce eith er pistillate or stam in ate flowers, or
vegetative grow th, or all th ree, w hile th e th ird bud
arising a t a given node a p p a re n tly does not d iffe re n tia te
flow ers and produces very few catkins w hen th e buds
above it are rem oved as late as M arch 24.
T he rem oval of m ore th a n one b u d a t nodes n e a r th e
term inal shifts fru iting possibilities to m ore basal buds
w ith a tendency to increase yields an d low er catkin p ro ­
duction, bu t w h ere only one bud is rem oved from nodes
n ear th e term inal it tends to sc atte r fru itin g a n d v eg e ta­
tive gro w th over th e entire shoot. It also has th e effect
of low ering production from 1.6 to 1.5 nuts p e r shoot, as
shown in T able 10, while th e n u m b er of catkins is re d u c ­
ed from 8.5 to 6.3 p er tre a te d shoot.
D isbudding as late as April 14 caused buds to fru it
th a t otherw ise would have rem ain ed la te n t or produced
only catkins and short vegetative shoots t h a t w ould have
abscised w hen th e catkins fell.
n p H E S E DISBUDDING experim ents indicate t h a t th e
condition existing in th e pecan is m ore closely co m p ar­
able to th a t existing in th e grap e, rasp b erry , an d p ro b ­
ably other b ram b le fruits th a n to t h a t found in th e apple,
peach, and most oth er tree fruits whose fru itin g c h a r ­
acteristics have been carefully studied.
In other words, a com paratively la rg e p erce n tag e of
th e over-wintering buds m ay be potentially pistillate
flow er buds. U nd er norm al circum stances only a few
of these will produce pistillate flow ering shoots. T he
rest rem ain d o rm an t or are abscised or p e rh a p s give rise
to w eak vegetative p a rts th a t abscise along w ith th e fa ll­
ing of th e catkins. They function only w h en those th a t
norm ally give rise to pistillate flow ers are rem oved by
62

pruning or oth er means. This is in m ark ed contrast to
th e condition existing in th e peach, pear, etc., w here the
flow er p arts are d ifferentiated in the bud during the
growing season of th e y ea r before which they open.
GENERAL DISCUSSION
HP HE D IFFER EN TIA TIO N of the stam inate flow er cluste r in th e bud is a process th a t extends over a co m para­
tively long period, beginning in early spring in the first
form ed buds of th e season and occurring in mid or late
sum m er in those buds laid down in the axils of late form ­
ed leaves on secondary shoots. The extreme earliness
with which these first catkins are differentiated (asso­
ciated as it is in time with very slight leaf development)
leads to the surmise th a t it is probably more closely re ­
lated to and d ep en d en t on food storage conditions in the
p aren t tw ig or branch, in the old w o o d : and, therefore,
on late sum m er and fall activities in th e tree th a n on
spring grow ing conditions. However, catkin differen­
tiation in th e bud is so ab u n d a n t and tak es place under
such a wide ran g e of environmental, nutritive, and growth
conditions t h a t obviously it seldom, if ever, becomes a
limiting fa c to r; and, therefore, for all practical p u r­
poses m ay be ignored.
Pistillate-flow er cluster differentiation, on th e other
hand, is not all-summer in duration but occurs within a
relatively short period as grow th is starting in th e spring.
The time of its occurrence is probably in itself sufficient
evidence th a t— to the extent th a t it is a response to nu tri­
tive conditions w ithin th e p lan t— it is due to or associated
with w inter storage of food m aterials. This means th a t
it is determ ined by w h a t goes on in th e tree during the
sum m er and fall before.
In other words, th e pistillate flow er crop of th e follow­
ing year is a p p a re n tly being determ ined while th e nuts of
the cu rrent y e a r are filling and m aturing, and it is then
th a t cultural and fertilizing practices are very im portant
if they are to function in increasing yields. F u rth e r evi­
dence on this point is supplied by the d ata from the de­
foliation and ringing experiments.
n r HE DISBUDDING and pruning (heading back) experim ents indicate a r a th e r m arked degree of flexibil­
ity in th e pecan. It obviously h as the ability to a d a p t it­
self to circumstances, in re g a rd to fruiting, by developing
63

a fru it crop from buds th a t w ould never h av e o pen ed h a d
th e buds th a t norm ally open been u n in ju red or u n re ­
moved. This probably m eans t h a t some types of p run in g
could be em ployed w ithout m a terially in te rferin g w ith
crop production. However, it is doubtful if th e evidence
available w a rra n ts th e interferen ce t h a t p ec an yields may
be increased practically or profitably by pruning.
T h ere is a m a rk e d correlation betw een ty p e and
am ount of new shoot grow th m ad e an d th e ten d en cy to
form pistillate-flow er producing shoots. W ith some v a­
rieties this correlation is close (i. e. th e ra n g e in g ro w th
associated w ith pistillate bud production is n a rro w ) ;
with others it is not so close.
T he real problem of th e g ro w er is to h a n d le his trees
in such a m an n er th a t each y ea r a com paratively larg e
p ercen tag e of th e ir shoot g ro w th will be as n early th e
optim um as possible for th e v ariety in question. This
m eans th a t control over production is possible larg ely
thro ug h th e soil and, incidentally, p artic u larly im p o rta n t
is it th a t cultural operations and fertilizatio n practices be
sufficient to assure good vegetative grow th.
SUMMARY
1. B u d s lo c a t e d a t th e m o r e b a s a l n o d e s o f t h e n e w s h o o t s
r a p id ly d i f f e r e n t i a t e in t o th r e e , s o m e t im e s f o u r , a n d o c c a s io n a lly
f i v e g r o w in g p o in ts u n d e r a c o m m o n b u d s c a le . E a c h o f t h e s e g r o w ­
in g p o in t s is s u r r o u n d e d b y a s e p a r a t e b u d s c a le .
2 . T h e g r o w in g p o in t o c c u p y in g t h e c e n t r a l p a r t o f t h e c o m ­
p o u n d b u d r e t a in s it s v e g e t a t i v e n a t u r e t h r o u g h o u t t h e g r o w in g s e a ­
s o n , w h ile t h e o th e r s r a p id ly d i f f e r e n t i a t e c a tk in s .
3 . A s t h e g r o w in g s e a s o n c o n t in u e s b u d s f o r m e d a t o t h e r n o d e s
u n d e r g o s im ila r c h a n g e s .
H o w e v e r , t o w a r d th e e n d o f th e s e a s o n
b u d s o n t h e te r m in a l p a r t o f t h e s h o o t d e v e lo p m o r e r a p id ly th a n
t h o s e n e a r it s b a s e .
U lt im a t e ly th e m o r e t e r m in a l b u d s c o m e to
h a v e m o r e a n d la r g e r c a t k in s t h a n t h o s e a t m o r e b a s a l n o d e s .
4 . C a tk in f lo w e r c lu s t e r s d e v e lo p m o r e r a p id ly in t h e b u d s o f
t h o s e v a r ie t ie s t h a t a r e h e a v y c a tk in p r o d u c e r s a n d t h a t p u t o u t
c a t k in f lo w e r s e a r ly in t h e s p r in g th a n in th e c a s e o f v a r ie t ie s t h a t
a r e lig h t c a tk in p r o d u c e r s .
5 . S h o o t s t h a t t e r m in a t e s p r in g g r o w th w it h a c lu s t e r o f n u t s
o r b y d r o p p in g t h e t e r m in a l b u d a n d t h a t la t e r m a k e a s e c o n d
g r o w th , d i f f e r e n t i a t e c a tk in f lo w e r s o n t h e “ s e c o n d g r o w t h ” p a r t
o f t h e s h o o t ( t h a t is , o n its la t e r a ls o r s e c o n d a r ie s ) v e r y m u c h a s
t h e y d id o n f ir s t g r o w th . T h e s e c o n s t it u t e t h e m a j o r it y o f th e c a t ­
k in s a p p e a r in g a s b lo s s o m s o n s u c h s h o o t s t h e f o llo w in g s p r in g .
T h e b a s a l c a t k in b u d s o n s e c o n d g r o w th s h o o t s a r e th e a p ic a l c a t k in
b u d s o f t h e f i r s t s h o o t g r o w th .
S u ch b u d s m a y d rop w h en th e
s e c o n d g r o w t h s h o o t p u s h e s o u t , o r r e m a in o n t h e n e w s h o o t u n t il
th e f o l l o w i n g s p r in g , a t w h ic h tim e t h e y m a y a b o r t o r p r o d u c e
f lo w e r s .
6 . T h e t r u e t e r m in a l b u d in m o s t v a r ie t ie s d o e s n o t u s u a lly d i f ­
f e r e n t i a t e a n d d e v e lo p c a tk in f lo w e r s .
7 . I f a t r e e is p r e m a t u r e ly d e f o lia t e d b e c a u s e o f in s e c t a t t a c k ,

64

d r o u g h t, s to r m s , o r o t h e r c a u s e s , a n d th e t r e e m a k e s a se c o n d
g r o w th , c a t k in f lo w e r s m a y a p p e a r a lo n g w ith t h e n e w g r o w th .
8 . T h e p a r t o f t h e c o m p o u n d b u d th a t r e m a in s v e g e t a t iv e w h ile
c a tk in f lo w e r b u d s a r e d if f e r e n t ia t in g c o n t in u e s its d e v e lo p m e n t
b y f o r m in g n o d e s , in t e r n o d e s , le a v e s a n d r u d im e n ta r y b u d s in th e
a x ils o f t h e le a v e s . B u d s to w a r d th e t e r m in a l p a r t o f th e s h o o t b e ­
c o m e v e r y m u c h la r g e r th a n th o s e to w a r d t h e b a s a l p a r t o f th e
s h o o t. In g e n e r a l th e a p ic a l b u d a t e a c h n o d e b e c o m e s la r g e r th a n
t h e o n e im m e d ia t e ly s u b t e n d in g it a t th e sa m e n o d e a n d th e s e c o n d
b u d a t a n o d e la r g e r th a n t h e o n e j u s t b e lo w it , a n d so o n w ith a ll
b u d s o c c u r r in g a t a n y g iv e n n o d e .
9 . J u s t a b o u t t h e tim e b u d s b e g in to s w e ll in th e s p r in g f o llo w ­
in g th e ir f o r m a t io n s o m e o f th e h ith e r to v e g e t a t iv e b u d s b e g in to
d i f f e r e n t i a t e p is t i l la t e p r im o r d ia . T h e s e c o n t in u e th e ir d e v e lo p m e n t
a s th e in t e r n o d e s o f t h e v e g e t a t iv e s h o o t e lo n g a t e u n t il a b o u t te n
or m o r e le a v e s h a v e u n f o ld e d , a t w h ic h tim e th e p is t illa t e flo w e r s
b e c o m e v is ib le o n th e te r m in a l p a r t o f th e y o u n g s h o o t.
1 0 . W h e n s e c o n d g r o w th o c c u r s o n a n y s h o o t t h e b u d s fo r m e d
to w a r d it s te r m in a l a r e th e o n e s th a t p r o d u c e th e m a jo r ity o f th e
n u ts o n th a t sh o o t t h e f o llo w in g y e a r .
1 1 . N o r m a lly t h e t e r m in a l b u d o f m o st v a r ie t ie s o f th e p e c a n
a b s c is e s a n d t h e p i s t illa t e flo w e r s a r e d if f e r e n t ia t e d in la r g e r a p ic a l
b u d s a t t h e n o d e s n e a r th e te r m in a l. H o w e v e r , a ll v a r ie t ie s stu d ie d
m a y h o ld a t le a s t a f e w te r m in a l b u d s a n d d i f f e r e n t i a t e p is t illa t e
flo w e r s in th e m , a n d s o m e v a r ie t ie s m a y fo r m a n d h o ld m a n y t e r ­
m in a l b u d s t h a t d i f f e r e n t i a t e p is t illa t e flo w e r s .
1 2 . T h e s h o o t s o f s o m e v a r ie t ie s , S u c c e s s f o r e x a m p le , m a y lo s e
a lm o s t a ll b u d s f o r m e d a t a ll n o d e s a t s o m e tim e o f th e y e a r . T h e r e
is a t e n d e n c y in s u c h c a s e s fo r th e s t r o n g b u d s a t n o d e s n e a r th e
te r m in a l p a r t o f th e s h o o t to b e r e t a in e d fo r f lo w e r a n d fr u it p r o ­
d u c tio n .
1 3 . T h e p e c a n p r o d u c e s a n u m b e r o f d if f e r e n t k in d s a n d le n g t h s
o f s h o o ts w h ic h b e h a v e s o m e w h a t d i f f e r e n t l y in th e s e v e r a l v a r ie tie s .
S u c h v a r ia t io n s s u g g e s t th a t o r c h a r d p r a c tic e s sh o u ld b e a d a p te d to
th e v a r ie t y in o r d e r th a t m a x im u m p r o d u c tio n m a y b e o b ta in e d .
1 4 . In g e n e r a l v e r y s h o r t a n d v e r y lo n g s h o o ts c a r r y v e r y f e w
n u ts .
1 5 . E a c h v a r ie t y p r o d u c e s th e m a x im u m n u m b e r o f n u ts on
s h o o ts o f r a t h e r d e f i n it e le n g t h .
1 6 . L o n g s h o o t s th a t f r u it p r o d u c e m o r e n u ts th a n s im ila r sh o r t
s h o o t s o n a g iv e n v a r ie t y .
1 7 . M a n y s h o o t s , e s p e c ia lly t h o s e th a t a r e w e a k , a b s c is e th e ir
c lu s t e r o f p is t i l l a t e f lo w e r s . W ith so m e th is a b s c is s io n o c c u r s b e f o r e
t h e c lu s t e r o f f lo w e r s is v is ib le , w ith o t h e r it o c c u r s a f t e r it is v is ­
ib le , b u t b e f o r e b e in g r e c e p t iv e to p o lle n .
1 8 . P r u n in g s h if t s v e g e t a t iv e g r o w th , c a tk in f lo w e r d e v e lo p m e n t
a n d p is t illa t e f lo w e r d if f e r e n t ia t io n to b u d s a t m o r e b a s a l n o d e s.
1 9 . D e f o lia t io n r e d u c e s c a tk in f lo w e r d e v e lo p m e n t a n d p is t illa t e
flo w e r b u d d i f f e r e n t i a t io n .
2 0 . R in s in g a n d a llo w in g th e r in g p a r t ly o r e n t ir e ly to h ea l
s c a t t e r s c a t k in f lo w e r d e v e lo p m e n t a n d p is t illa t e f lo w e r b u d d i f ­
f e r e n t ia t io n o v e r t h e t r e a t e d s h o o t.
2 1 . R in g in g a n d p r e v e n t in g t h e h e a lin g o f th e r in g s to p s c a tk in
f lo w e r d e v e lo p m e n t a n d p r e v e n ts p is t illa t e f lo w e r b u d d i f f e r e n t ia ­
tio n a b o v e t h e r in g .
2 2 . R e m o v in g f ir s t a n d s e c o n d a p ic a l b u d s a t n o d e s n e a r t h e t e r ­
m in a l p a r t s o f a s h o o t in g e n e r a l s h if t s c a tk in f lo w e r d e v e lo p m e n t
a n d p i s t il l a t e f lo w e r d if f e r e n t ia t io n to b u d s a t m o r e b a s a l n o d e s,
s h o w in g t h a t t h e th ir d a p ic a l b u d a t a n o d e d o e s n o t u s u a lly h a v e
p is t illa t e f lo w e r p r o d u c in g p o s s ib ilit ie s .

65

ACKNOW LEDGM ENTS
n r HE W RITER wishes to express his th a n k s to th e authorities of th e A lab am a Polytechnic Institute and
Michigan State College fo r m aking this investigation pos­
sible th ro u g h th e ir cooperation; to th e several m em bers
of th e Botanical and H orticultural D ep artm en ts of both
institutions who have furnished h elp ful suggestions and
criticism ; to Prof. V. R. G ard n er especially for suggesting
th e problem and furnishing general g u id a n c e ; to Prof. F.
C. B radfo rd and Dr. J. W. Crist who have fu rn ish ed spe­
cial guidance in obtaining and classifying these data.

66

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68