I.
INTRODUCTION
The ramified shoot system of an angiosperm
comes into being due to histogenesis and organogenesis,
the two dynamic characters of shoot apex.
The shoot apex
is the continuation of embyyogeny as these characters are
I
inherent in it, since its inception in the embryo.
The
versatile nature of form and function of the shoot is due
mainly to its adaptation, in response to the prevailing
necessity dictated by the environment.
This exmorphic
diversity of the lateral branches, apart from their varying
size and form, is demonstrated
in the form of tendrils,
1(
thorns, phylloclades etc.
Hence for a better understanding
of their nature, ontogenic studies of. the shoot apex are
pre-requisite.
The encouraging amount of literature on the
shoot apex, accumulated during the present century demonstra'
tes sustained efforts to understand the mysteries underlying
its morphogenesis.
2
Shoot apex
The tunica-corpus theory (Schmidt, 1924) has
replaced Hanstein's histogen theory d86g) because of
the former's feasibility and plasticity in interpreting
the angiosperm shoot apices.
Actually the tunica-corpus
concept is a modified version of Hanstein's histogen
theory without its histological predestination.
The
concept of cytohistological zonation (Gifford, 1950, 1954)
and plastochronic variations have further contributed to
our understanding of structure and organization of the
shoot apex.
But the role of the shoot apex in the development
of the shoot is not fully understood.
Whether the shoot
apex is a self-determining and dominant centre of
development or it is a plastic region acting under the
stimulus from the mature subjacent tissues, is to be
investigated (Allsopp, 1964).
Subsequently Dermen (1953) and Popham and Chan
(1950, 1952) respectively proposed the concepts of ‘primary
histogenic layers* and 'mantle-core' as substitutes for
tunica-corpus theory.
t
3
A c c o r d i n g to t h e F r e n c h s c h o o l
1947; B u v a t ,
1951?
1952)
(Plantefol,
t h e r e is a z o n e o f
initial3 in the shoot apex.
The
centres
' l ’a n n e a u
for the
inception
*
of the
foliar p r i m o r d i a are l a i d d o w n in t h i s region.
the terminal
d*
attente
(1954)
region of the shoot apex a zone of meristeme
is p r e s e n t .
rejected the
( 1952) arguing that
It
is a w a i t i n g m e r i s t e m .
concepts of Plantefol
equally potentially meristematic.
n o t s u b s c r i b e to t h e
t h e y g i v e r i s e to,
w h e r e a n a p e x is
(b)
i n i t i a t i o n zone;
on e zone o f
But
form the origins
(Nougarede,
a c c o r d i n g to B a l l
cell/cells
into
(a)
where an apex shows superficial
(c) m o n o p i e x ,
initiation
1 9 6 7 ).
interpreted in terms of tunica-
simplex,
pattern of divisions
no
together with
( 1961) classified shoot apical meristems
theory;
does
necessijty w h e t h e r
T h e s e initials,
o f the different tissues of the stems
corpus
( 1 9 6 5 ) also
existence o f apical
i n t h e s h o o t a p e x is a l o g i c a l
independent layers
duplex,
its cells a re
Wardlaw
( 19 6 1 ) the
t h e y a r e d i s c e r n i b l e o r not.
Newman
and Buvat
concept o f t h e F r e n c h school.
A c c o r d i n g to N e w m a n
the
(1947)
Gifford
interdependence of different growth
reg i o n s o f the shoot apex shows tha t all
initials
At
where shoot
c o n f i n e d to
( 1959s*
apical
1960b-)
apex has only
cell o r
cells.
a s t h e r e i s no r e g u l a r
in the superficial
c e l l s o f s h o o t apex,
c a n b e r e c o g n i z e d as s u p e r f i c i a l
initials.
4
P Ia sto c h ro n ic v a r ia tio n s ;
A ccording to Abbes
and Phinney (195D> th e p o rtio n d i s t a l to th e youngest
l e a f primordium i s th e sh o o t apex.
V arious p la s to c h ro n ic
phases have been named by th e s e workers as e a rly su b stag e,
mid su b stag e and l a t e su b sta g e .
P a o lillo and G if trd
(1961) r e f e r r e d to d i f f e r e n t phases o f p la sto c h ro n as
pre-m axim al, maximal, m inim al, e a r ly post-m inim al and l a t e
post-m inim al depending upon th e r a t i o o f th e h e ig h t o f th e
shoot apex and le n g th o f th e youngest l e a f primordium.
R ouffa and Gunckel (1951--) and M illin g to n and Gunckel (1950)
observed f lu c tu a tio n s in th e number o f tu n ic a la y e r s d u rin g
a p la sto c h ro n b u t d id n o t co n sid er them as s i g n i f i c a n t .
Ontogeny o f shoot apex :
The s tu d ie s in th e ontogeny
o f th e sh o o t apex a re e s s e n tia l as i t i s th e c e n tre o f
a c t i v i t y and m ajor c o n tr o llin g re g io n o f th e p la n t.
The
s tu d ie s in th e o r ig in and development o f fu n c tio n in g a p ic a l
m eristem were c a r r ie d o u t by Reeve on G arrya and Pisum
(1942, 1948a, 1948b), G iffo rd and Tepper on Chenopodium
album (1962), S e id lo v a , Horavka, O partna and Krekule on
Senecio v u lg a ris (1964-) and, Nougarede, G iffo rd and Rondet
on Amaranthus re tro fL e x u s (1965)*
5
A x i l l a r y buds
T h e a x i l l a r y b u d is t h e p r o g e n i t o r o f t h e
lateral
branch,
which organizes
It arises
into
as a m e r i s t e m a t i c t issue
a s h o o t apex.
differentiates
as a l a t e r a l
reproductive.
Schascht
relation
( 1853)
either vegetative or
studying the
chronological
concludes that the initiation o f the bud
that o f t he axillant leaf.
equal
branch
It u l t i m a t e l y
morphological
status
Hofmeister
(186 8 )
to t h e l e a v e s
follows
attributes
and braiches
and
infers that the secondary axes originate along certain
areas
of the vegetative shoot
apex.
with Schascht regarding the time o f
lateral
that
the buds
especially
Koch
Goebel
(1905)
( 1893 ) r e c o r d s
in S y ringa v u l g a r i s ,
are initiated at the third or
t h e s h o o t apex.
( 1870) a g r e e s
initiation of the
b u d a n d its s u b t e n d i n g leaf.
in m any angiosperms,
derived
Sachs
fourth node behind
reports t h a t the buds
are
from the terminal meristem and they sustain their
e m b r y o n i c traits.
Esau
to
( 1965 a ) p r e f e r s
'axillary'
the use o f the term
to d e s i g n a t e t h e b u d a s t h e l a t t e r d o e s
indicate the locus of origin d f
originates
lateral*
at the apical
t h e bud.
m e r i s t e m a n d as
Commonly the bud
a result of
s u b s e q u e n t g r o w t h r e - a d j u s t m e n t b e c o m e s s h i f t e d to
a x i llary position.
not
the
6
The main problem connected with the morphogenesis
of axillary buds is whether the origin can be traced back
to the meristematic shoot apex or to the occasional renewal
of the meristematic activity by the already differentiated
tissue.
Kundu, Rao and Saha (1954) suggested that the
following features should be attended to in studying the
ontogenetical aspects of the lateral buds.
(1)
Whether the early bud meristem is a component
of foliar or stem tissue.
(2)
. The chronological relationship between the
initiation of the bud and its axillant leaf.
(3)
Whether the bud meristem is initiated from the
primordial meristem or from the vacuolated cells situated
few nodes behind the shoot apex by dedifferentiation and
redifferentiation.
(4)
The original place of initiation.
(5)
The final place of disposition.
In.addition to the above aspects I consider that the
following features also require attention.
(1)
Whether the bud has the procambial connection
from the earliest recognizable stage of differentiation or it
7
develops late.
(2)
Whether the differentiation of procambium
occurs acropetally or basipetally or in both directions.
(3)
Whether the procambial differentiation occurs
before or after the prophyll initiation.
(4)
The number of prophylls and their
vascularization.
(5)
Whether the procambial supply to the bud is
from the axill'ant leaf trace or other portion of the axial
vasculature.
There are two schools of thought regarding the origin
of the bud.
According to one view the bud meristem is
derived from the shoot apex (Koch, 1893; Schmidt,
Louis,
1935, Wardlaw, 1943; Miller and Wetmore,
Garrison,
1949a, 1949b, 1955* Gifford,
1946;
l95l,b;Kundu et al.,
1954; Shah, 1 9 6 O; Shah, Poulose and Unnikrishnan,
Kamji and Parameswaran,
1961; Tucker,
1924;
1967;
1963; Marushige,
1 9 6 5 ).
It becomes detached from the shoot apex in the subsequent
nodes due to the differentiation of surrounding cells.
has been designated as a ‘detached meristem’ (Wardlaw,
It
1943).
According to the other view the bud originates from a tissue
away from the shoot apex by dedifferentiation and
8
r e d i f f e r e n t i a t i o n o f a lre a d y v a c u o la te d c e l l s
( P r ie s tle y and Sw ingle, 1929; Majumdar and D a tta ,
194S; Kundu and Rao, 1952, 1955)*
A ccording to Ramji
and Parameswaran (1 961) th e buds o r ig in a tin g in s it u a ti o n s
o th e r th a n th e a x i ls sh o u ld be r e f e r r e d to as a d v e n titio u s
buds.
However, th e y ag ree t h a t in c e r ta in t r e e s where- th e
i n i t i a t i o n o f buds i s d e fe rre d th e bud m eristem o r ig in a te s
by d e d if f e r e n tia tio n and r e d i f f e r e n t i a t i o n .
The term 's h e l l zone* has been in te r p r e te d v a rio u s ly
by v a rio u s au th o rs (G iffo rd , 195lf>;Esau, 1953, 1965o.jSh.ah,
1960j Shah e t a l . ,
19&7; Tucker, 1963; M arushige, 1965).
But i t s o r ig in and r o le in th e ontogeny o f th e bud r e q u ir e
f u r th e r in v e s tig a tio n .
A bud may have one o r two p ro p h y lls .
The term
'p ro p h y ll* i s a p p lie d in th e p re s e n t in v e s tig a tio n to th e
f i r s t l e a f o r le a v e s borne by an a x i l l a r y bud, i f th e y a re
p re s e n t a t th e base o f th e bud and a re sm a lle r in s iz e th an
th e normal f o lia g e le a v e s .
The ontogeny o f a v e g e ta tiv e d e te rm in a te shoot apex
i s a l i t t l e ex p lo red a re a in th e f i e l d o f anatom ical re se a rc h .
/
The d eterm in ate a p ic a l m eristem s commonly become p o in te d a t
m a tu rity .
The p o in te d s tr u c tu r e s o f p la n ts have been
9
d e sig n a te d as p r ic k le s , sp in e s and th o rn s ;
e s p e c ia lly th e te£ms 's p in e ' and 'th o r n ' have been
v e ry lo o s e ly used.
In t h i s th e s is th e term 'th o r n ' is
used f o r a lo n g p o in te d s tr u c t u r e , re p re s e n tin g a l a t e r a l
o r a x i l l a r y a x is .
The th o rn sh o o ts p ro v id e an o p p o rtu n ity
to stu d y th e g rad u al c e s s a tio n o f th e m e ristem atic a c t i v i t y
and t h e i r tra n sfo rm a tio n in to p o in te d o rg an s.
The s tu d ie s
in t h i s f i e l d a re v ery few (DaLbrouck, 1875; Schube , 1885;
A u b erto l, 1910; Uphof, 1925; Schnee, 1939; T r o ll, 1935r
1939, 1948; VariDerpi$L, 1951)*
The l i t e r a t u r e on th e
th o rn s d e a ls w ith a com parative account o f norm al,
unm odified and u n d eterm in ate l a t e r a l sh o o ts and, h ig h ly
su p p ressed , m odified and d e te rm in a te th o rn sh o o ts.
T ro ll
(1935-1939, 1948) in th e reco n n a issa n ce o f l i t e r a t u r e on
th o rn s in f e r r e d , t h a t th e r e was a g rad u al d ec re ase in th e
m e riste m a tic a c t i v i t y a t th e th o rn jtip .
He concluded t h a t
th e th o rn sh o o ts can be reckoned as p reco cio u s s h o r t sh o o ts.
Delbrouck ( 187S) and T ro ll (1948) sug g ested , t h a t th e th o rn
sh o o ts may o r ig in a te from normal a x i l l a r y buds, supernum erary
buds a n d /o r te rm in a l buds.
Thorns may b ea r s c a ly an d /o r
f o lia g e le a v e s .
B la se r (1956) worked o u t th e ontogeny o f th o rn
sh o o ts and t h e i r a b o rtio n in G le d its ia tr ia c a n th o s .
and Mehra (1963) w h ile stu d y in g th e nodal anatomy o f
P ant
10
B ougainvillea glabra and B. spectabilis, and Pal
(i9 6 0 ) while investigating the branching system of
Carissa carandas refers that the thorns are the modifi­
cations of axillary buds.
studied.
in
Bu t their development is not
( 196©) concludes that the branching system
Pal
carandas is of forking type and it varies from di- to
tetra-chotomy.
H e believes that due to forking a branch
does not have shoot apex at-maturity.
Procambialization of bud :
A n axillary bud at its
inception may be connected to the vascular supply of the
main axis by the bud traces as in Phlox drummondii
and Wetmore,
1946).
(Miller
It m a y also establish connection with
the main axis later in the development as in Michelia fuscata
19 63).
(Tucker,
In Hibiscus cannabinus (Kundu and Rao,
1955) the earliest bud has residual meristematic connection.
In M a g n o l i a , L i r i o d e n d r o n , Alnus and Schizandra also the
earliest bud meristem'has residual meristematic connection
(Garrison,
1955)*
There are three types of differentiation
of.procambium to the bud based on the direction of
differentiation; acropetal differentiation (Reeve. , 194&;
Sterling,
1949b,
Tucker,
1945; Miller and Wetmore,
1955; Sussex,
1 9 4 6 ; Garrison,
1955; Kundu and Rao,
1955>
1949a,
1957;
19 6 3 ), basipetal differentiation (Majumdar and Datta,
19 46) and both basipetal and acropetal differentiation
11
(Gifford,
Clowes
195lb}Ramji
(19^1)
mentions
about basipetal
and Parameswaran,
that there
1 9 6 1 ).
is n o t h i n g o d d
differentiation of procambium although
the conventional
c o u r s e is acropetal.
H e concludes
that the state of differentiation of parent axis
at the
t i m e o f b u d i n i t i a t i o n d i c t a t e s w h e t h e r a b u d is to be
supplied with procambium basipetally or acropetally.
The term
connotations.
'residual meristem'
Esau
(1953)
considers
meristematic tissue and less
vascular
(19 5 5 )
system,
has different
that
it is a
determined part of potential
del a y e d in its differentiation.
u s e d i t to d e s i g n a t e t h e
e a r l y b u d trace.
Kundu and Rao
Garrison
early bud meristem and the
( 1 9 5 5 ) u s e d t h e t e r m to
denote intermediate stage in the differentiation of
p r o c a m b i u m from meristem.
cell s .
In the present
g a t i o n t h e t e r m “r e s i d u a l
meristem” has
investi­
b e e n used,
as
d e f i n e d b y Esau.
Miller and Wetmore
and Kundu and Rao
s e m i l u n a r arcs;
( 1 9 55)
(1946),
histogenic
( 1949b,
describe the bud traces
1955)
as
crescentic arcs or horse-shoe shaped or
ring shaped at different levels
considers
Garrison
f r o m t h e node.
that these varying configurations
expressions
Shah
( 1968)
are the phasic
of the gradual differentiation of
12
t h e p r i m a r y v a s c u l a r s y s t e m o f t h e bud.
( 1 9 4 9 b)
Garrison
r e p o r t s t h e f o r m a t i o n o f p r e c o c i o u s procarabium
f o r t h e l e a f o f t h e bud.
Shah
(1968)
records that bud
traces are differentiated from the axillant l e a f trace
as branches
formed by direct differentiation of the
m e r istematic cells
Syzygium.
T hey may
t h r o u g h residual m e r i s t e m as
also
and redifferentiation of
in
differentiate by dedifferentiation
certain sectors
of ground meristem
i n t e r v e n i n g be t w e e n t h e b u d a n d t h e l e a f t r ace as
in
Clerodendrum.
The organization o f the primary vascular system
in the developing b ud o r branch constitutes
aspect of bud morphogenesis,
as
it p a r t l y r e f l e c t s t h e
vascular organization of the main axis
concept o f leaf traces
implies that
axial vasculature develops
(Esau,
1965a).
of a s h o o t axis
an important
(Esau,
1965a).
The
at l e ast a p a r t of the
in d i r e c t r e l a t i o n to t h e l e a v e s
In
angiosperms the primary vascular
is
closely associated with the leaves and
is o f t e n d e s c r i b e d a s a s y s t e m o f
1943
, 1965a,
systsn
interconnected leaf
traces
(Esau,
traces
in t h e f o r m ation o f t h e p r i m a r y v a s c u l a r s y s t e m o f
t h e b r a n c h is a l i t t l e
differentiation.
1965b).
The role of the bud
explored aspect of vascular
:
13
Two basic types o f vascular interconnections
are recognised.
They are "open" and "closed"
1 9 4 5 , 1 9 5 4 ; Ezelarab and Dormer,
(Dormer,
1963) or "sympodial"
and "reticulate" types (Philipson and Balfour,
1963).
There
are two views regarding the nature of primary vascular
system in the dicot stem.
(1) It forms a continuous
cylinder perforated by l eaf and branch gaps.
as a system of discrete bundles
1963).
Eames and MacDaniels
(2 ) It arises
(Philipson and Balfour,
( 1 9 4 7 ) visualize a common gap
for the branch and the l e a f or an independent gap for the
branch in dicotyledons.
The primary vascular system is
still described in terms o f leaf bundles and stem bundles
(Dormer,
1 9 4 5 ; Ezelarab and Dormer,
19 & 3 ).
It is not
absolutely dependent upon the presence of leaves
(Philipson,
1 9 4 9 ; Wardlaw,
1 9 5 0 )*
Esau (1965a), Tucker
(1963) and Jensen (1968) reported relationship between
phyllotaxy and the intervals at which l eaf traces are
interconnected to each sympodium.
Jensen (1968) also
describes a type of primary vascular system intermediate
and
between open and closed types.
Philipson
P h i l i p s o n A(1908) studied
the organization of primary vascular system in Rhododendron.
Dicotyledons have commonly two branch traces
s connecting the branch to the stem.
The branch trace is
defined as a vascular bundle in the main stem extending
14
between
its
connection with the vascular tissue of
the branch and that with
the main stem
(Es a u ,
the another vascular unit of
1965b).
It c a n also
b e d e f i n e d as
t h e s t r a n d o f v a s c u l a r t i s s u e d i f f e r e n t i a t e d to o r
t h e a x i a l v a s c u l a t u r e to o r
f r o m t h e bud,
thus l i n k i n g
t h e p r i m a r y v a s c u l a r s y s t e m s o f b u d a n d axis.
and Dormer
from
Ezelarab
classified five types o f bud trace connections
in t h e R a n u n c u l a c e a e
( 1963).
the bud trace relations
W i t h a d e s i r e to
T h e y h a v e also w o r k e d o u t
in Rt^adales
(1966).
c o n t r i b u t e to t h e v a r i o u s
o f shoot morphogenesis l i k e origin,
structure and
o r g a n i z a t i o n o f v e g e t a t i v e shoot apex,
ment o f axillary buds
facets
origin and develop­
and their procambialization,
h i s t o g e n e s i s o f b u d into
thorn,
development of the primary
vascular systen o f the bud and v a scular relationship o f the
bud,
axis
a nd leaf,
investigation on the following plants
has b e e n undertaken.
1)
Maytenus
2)
Carissa congesta Wight
3)
E h r e t i a b u x i f o l i a Ro x b .
4)
P l a n t a g o o v a t a Forsk.
5)
A e g L e m a r m e l o s Corr.
emarginata
(Willd.) Ding-Hou
15
The histogenesis of bud
b e e n d e s c r i b e d i n M.
into a t h o r n has
e m a r g i n a t a , C.
congesta and
A.
marmelos. The abortion of shoot apex
is
investigated.
C.
c o n g e s t a a n d A.
congesta
The origin and development of leaves
h a v e b e e n s t u d i e d i n M.
Anatomy of thorns
i n C.
e m a r g i n a t a a n d E.
is a l s o
dealt with
i n M.
buxifolia.
emarginata,
marmelos.
i
The ontogeny of shoot apex and branching pattern
have been worked out
i n P.
o v a t a , a plant of great economic
i m p o r t a n c e to t h e G u j a r a t S t a t e .