J.
G. VAUGHAN: INVESTIGATION INTO STEM APEX OF SYRINGA VULGARIS
L.
5ll
to
An investiga tion into the stem apex of Syringa vulgaris L. with special reference
Chelsea
,
VAUGHAN
G.
J.
By
um.
procambi
leaf
and
leaf
the
of
n
.formatio
the
Polytechn ic.
(With Plates 24 and 25)
[Read 18 January 1951.]
INTRODUCTION
Philipson (194fJ) has recently reviewed the ideas of plant morpholo gists on the arrangement of cells in the stem apex. He explains, in some detail, the tunica-co rpus concept.
and an
According to this concept the stem apex consists of two regions- an outer tunica
In the
divisions.
cell
of
direction
the
to
according
differ
regions
inner corpus. The two
cell
corpus,
the
in
while
surface,
the
to
angles
right
at
place
take
divisions
cell
tunica,
apex
stem
of
review
a
in
divisions take place in a number of direction s. Sitton (1944),
is one
morpholo gy, shows how the depth of the tunica varies-i n Triticurn and Avena it
iburnurn
V
in
while
three,
rternisia
A
in
two;
orus
M
and
dron
layer deep ; in Carya, Rhododen
it is four. Reeve (1948), however, calls for care in regarding the tunica-co rpus concept
of
as a fixed one, and claims to have demonstr ated fluctuatio ns in the depth of the tunica
in
ed
establish
well
is
concept
this
less,
a number of plants, particula rly Salix. Neverthe
of
ation
interpret
another
of
writes
also
(1949)
modern studies of stem apices. Philipson
the stem apex-an interpret ation based on cell size and density of protoplas mic contents.
and a
According to this the stem apex consists of a central zone of lightly staining cells
a
also
is
There
.
meristem
flank
periphera l zone of smaller, more darkly staining cells, the
this
supports
(1942)
r
Majumda
others
Among
'file meristem ' which gives rise to the pith.
concept.
have
The position of initiation of the leaf seems to vary among the various genera that
studied,
plants
most
in
that,
been studied. Engard (1944) reviews this subject and states
(1935)
the leaf appears to originate in the second and third layers of the apex. Louis
conthe
d
introduce
he
when
gy
morpholo
apex
stem
to
ion
made a valuable contribut
the
which
on
base
atic
meristem
the
',
buttress
'foliar
or
n'
foundatio
'foliar
ception of the
this
d
supporte
has
leaf develops and which later forms part of the axis. Majumda r (1942)
idea.
is
Perhaps one of the most interestin g problems bearing on the study of the stem apex
sothe
leaf,
the
of
supply
vascular
the
to
rise
gives
which
tissue
the
of
the developm ent
shed
called leaf procambi um or leaf provascu lar meristem . This tissue is usually distingui
of
contents
the
cells,
elongated
of
from surround ing tissues by the fact that it consists
base
the
at
s
originate
um
procambi
the
that
is
belief
which stain deeply. Quite a common
ly
of a leaf primordi um and then develops acropetal ly into the growing leaf and basipetal
(1944)
Sifton
plant.
the
of
system
vascular
general
the
join
to
stem
the
of
axis
into the
Flot
summariz es the views of some workers who have supporte d this idea. They are
(1927),
Langdon
(1927),
Lange
(1921),
(1905, 1906, 1907), Herrig (1915), Bugnon
many
Yarbroug h (1934), Foster (1935), and Cross (1936, 1937). In more recent years
times,
all
at
um
procambi
of
y
continuit
the
ated
demonstr
have
to
claimed
workers have
that is, the procam bium supply of a particula r leaf is continuou s with the general vascular
supply of the plant from the moment of initiation of the leaf. These investiga tors include
(1942,
Priestley , Scott & Gillett (1935), Boke (1941), Esau (1942), Majumda r (1942), Reeve
recent
very
this
Bearing
(1949).
Garrison
and
(1948)
Lawalree
(1944),
1948), Engard
conwork in mind, it is therefore surprisin g to find that Ball (1949) supports the oldish
intro(1935)
Louis
ly.
basipetal
and
ly
ception of leaf procambi um developin g acropetal
It was
duced the term 'prodesm ogen' for tissue that gave rise to typical procambi um.
on
vacuolati
by
defined
was
and
meristem
apical
the
with
ion
constitut
in
itself identical
512
J. G. VAUGHAN: INVESTIGATION IN'fO STEM APEX OF SYRINGA VULGARIS L.
and enlargement of surrounding cells. Helm (1931) and Kaplan (1937) held similar ideas.
Later workers, however (Esau, 1942; Engard, 1944), have not been able to establish the
existence of prodesmogen in the plants they have studied. After its initiation the leaf
develops into a tiny protuberance, the primordium, on the surface of the apex. For a very
short time the primordium consists entirely of meristematic cells with dense protoplasmic
contents. However, the cells of the primordium soon undergo vacuolation and enlargement. It is during this process that the leaf procambium becomes most apparent.
Vacuolation appears to start in the abaxial half of the leaf primordium and is followed by
vacuolation in the adaxial half (Majumdar, 1942; Reeve, 1942; Engard, 1944; Miller &
Wetmore, 1946; Philipson, 1947). During its early history the leaf primordium has a wellmarked apical meristem (Reeve, 1942; Engard, 1944; Bake, 1947).
MATERIAL AND METHODS
Syringa vulgaris L. was used for this investigation because it could be obtained easily
and its opposite leaves were favourable for sectioning.
Vegetative buds were collected at regular intervals over the period October 1948August 1949. They were collected from one tree in Ravenscourt Park and fixed on the
spot in formalin acetic alcohol.
The results about to be described are based on observations made on sections of buds
fixed in May 1949. The buds were embedded in paraffin wax and serial sections were cut
at thicknesses of 6, 8 and IO,u. Observations were made mainly on sections 8,u thick.
Both longitudinal and transverse sections were prepared. Longitudinal sections were
found to be the more useful. This investigation involved the sectioning of about 200 buds.
Various combinations of stains were employed, and Heidenhain's iron haematoxylin
alone was most satisfactory.
Photographs were taken through a lin. parachromatic objective with a Leitz 'Mikam ·
eyepiece camera. Slow process plates were employed.
An account of the observations which have been made on Syringa can be given in a
convenient form by the following description and figures. These figures have been made
from a selected series of preparations which provide a clear demonstration of the point of
view adopted in this paper.
Pl. 24, fig. I, represents a median longitudinal section S,u thick through t.he stem apex
of Syringa. The bases of two leaf primordia are shown, the left containing procambium.
The apex is almost flat and is approximately IOO,u wide. Using the tunica-corpus concept
there appears to be one layer of cells in the tunica and approximately five layers of cells
in the corpus. Cell divisions in the tunica are anticlinal, while those in the corpus are
periclinal and in other planes. The tunica sometimes seems to consist of two layers of
eells, but periclinal divisions in the second layer are fairly frequent. Considering the apex
from the point of view of the size of the cells and the density of the cell contents, there is
a well-marked central zone and a flank meristem of smaller more darkly staining cells.
A file meristem, with cells in vertical rows, giving rise to the pith is clearly present.
Between the cent.ra,l zone and the file meristem there is an intermediate zone of very much
flattened cells. The depth of the apex from the surface to the intermediate zone is about 60 p.
Pl. 24, fig. 2, represents a longitudinal flection 8,u thick through <t stem apex. The section
is slightly oblique. Only part of the apex iR shown. The important feature is the preAence
of a very small leaf primordium. H is clear that there has been much meristematic
activity in the flank meristem. The small leaf primordium seems to have been raised by
coil divisions, in various planes, in the two layers of cells underneath the tunica. The
tunica, covering the primordium, still only undergoes divisions at right angles to the
surface. Underneath the primordium there is to be seen the foliar foundation. There is
a well-defined procambium strand supplying the foliar foundation and the leaf primordium. This strand is continuous with the general vascular supply of the bud and, as one
may regard this eondition as about the earliest stage in the development of the leaf
J. G. VAUGHAN: INVESTIGATION INTO STEM APEX OF SYRINGA VULGARIS L.
513
primordium, it would seem that the procambial supply is continuous at all times. An
oblique section such as this is usually more useful for tracing the vascular supply of a bud
than a truly median one (see below). The procambium has been produced by longitudinal
divisions in the inner part of the flank meristem. It is difficult to determine the exact
point where the procambium strand stops in the apex. As one follows the strand up into
the apical meristem the procambium cells become shorter, and in the uppermost levels of
the strand the protoplasmic density of these cells is the same as that of the ordinary
apical meristem cells. To illustrate this point, at level A-B, procambium cells can be
seen running between ordinary apical meristem cells. Farther down the strand the procambium cells contrast sttongly with the pith and cortical cells as the result of their dense
protoplasmic contents and elongated nature-level C-D.
Pl. 25, fig. 3, represents a median longitudinal section 8p thick of a leaf primordium a
little older than the previous one. The primordium is approximately 135p high, and the
section shows it curved over the apex. The tissue of this primordium can be regarded as
being entirely meristematic, although the density of cell contents varies throughout the
organ. There is a well-marked meristem at the tip of this primordium. The density of the
protoplasmic contents of the cells that constitute this primordium apical meristem is the
same as that of the cells that constitute the stem apical meristem. Procambium tissue is
to be seen in the primordium, although it is not continuous in this particular section with
the rest of the vascular system of the bud. However, a connexion can be established in a
section further along the series. This apparent break in the continuity of the procambium
is due to the fact that the strand runs obliquely into the leaf and, consequently, a truly
median longitudinal section would show no continuity. Tangential longitudinal sections
demonstrate that the procambium strand runs obliquely. The cells that make up the
procambium have the usual elongated shape, and their contents stain just as deeply as
those of the cells that make up the two apical meristems already mentioned. There is a
connexion between the procambium and the leaf apical meristem, so it can be assumed
that this meristem is responsible for producing new procambium as the leaf grows. The
cells of the adaxial and abaxial tissues of the primordium have quite dense contents,
although the density is not that of the two apical meristems and the procambium already
mentioned. Like the procambium both adaxial and abaxial tissues, at this stage, are
derived from the apical meristem of the primordium. The tunica is still to be seen covering
the primordium, its dense cells undergoing anticlinal divisions. There is an indication of
vacuolation in the abaxial tissue of what must be regarded as the foliar foundation. In the
section there is to be seen a primordium of an axillary bud.
Pl. 25, fig. 4, represents an oblique longitudinal section 8p thick through a still older
primordium. It is still curved over the stem apex and is approximately 156p high.
Procambium tissue is to be seen in the primordium. Continuity of this tissue with the
rest of the vascular tissue of the bud is also obvious, the section being off the median plane.
The cells that make up the surface layer of the primordium have dense protoplasmic
contents, so that they can be assumed to be meristematic, undergoing divisions in the
anticlinal plane. All these cells have originated from tunica of the stem apex. The tissues
of the primordium have now undergone quite considerable vacuolation, this vacuolation
having extended farther up the abaxial side of the primordium than the adaxial side.
A median longitudinal section through a somewhat later stage showed the leaf primordium standing nearly upright and about 240 p, tall. Its apical meristem which was
responsible for the growth in length of the primordium was still clearly visible. The procambium was present, but its continuity could not be demonstrated because the section
was median. The cells of the primordium showed extensive vacuolation. The cells of the
covering layer of the primordium, the primordial apical meristem and the procambium
still had dense contents, but the rest of its tissues were quite highly vacuolated although
mitotic divisions were still to be observed.
The observations described above were confirmed in transverse sections.
514
J. G. VAUGHAN: INVESTIGATIO N IN'I'O STEM APEX Olf SYRINGA VULGARIS L.
CONCLUSION AND DISCUSSION
Nearly 200 buds of Syringa vulgaris L. have been sectioned, and examination of these
sections has shown that the stem apex of this plant can be divided, according to the
direction of cell divisions, into two regions-an outer tunica and an inner corpus. The
tunica consistR of one layer of cells showing anticlinal divisions while the corpus consists of
about five layers of cells showing periclinal divisions. Esau (1942) reports a similar
condition in Linum. On the basis of cell size and density of cell contents the stem apex of
Syringa shows the central zone, flank meristem and file meristem reported by various
workers for other plants (Majumdar, 1942; Philipson, 1947).
Leaf initiation appears to take place in the outermost two layers of the corpus which,
according to Engard (1944), is the most common position in the various plants that have
been studied. When extremely young leaf primordia are examined, procambium is always
to be found in close proximity. This procambium is produced by longitudinal divisions in
the inner region of the corpus. There is no evidence at all to support the existence of the
prodesmogen tissue of Louis (1935). Examination of numerous series of sections never
reveals a break in the procambial network, and it therefore follows that the vascular
system of the stem and leaf is continuous at all times. The leaf primordium develops on the
foliar foundation with the procambium strand following into it. This gives very strong
support to the work of such morphologis ts as Priestley et al. (1935), Boke (1941), Esau
(1942), Majumdar (1942), Reeve (1942, 1948), Engard (1944), Lawalree (1948) and
Garrison (1949); and is in opposition to the work of Flot (1905, 1906, 1907), Herrig (1915),
Bugnon (1921), Lange (1927), Langdon (1927), Yarbrough (1934), Foster (1935), Cross
(1936, 1937) and Ball (1949).
It has been noted in the introduction that a number of workers have described the
progress of vacuolation in the developing leaf, and that others have shown that the young
leaf grows in length by the activity of an apical meristem. The work on Syringa now
described is in full agreement with these other investigation s.
SuMMARY
1. The stem apex of Syringa vulgaris L. has been examined.
2. The apex oan be divided into a tunica and a corpus.
3. Under another scheme of classification the apex can be divided into a central zone,
a flank meristem and a file meristem.
4. Leaves are initiated in the outer layers of the corpus.
5. Procambium is produced by longitudinal divisions of cells in the inner layers of the
corpus.
6. The procambium supply of a leaf primordium is continuous at all times.
7. Vacuolation of the leaf primordium tissues is first abaxial and then adaxial.
8. The leaf primordium possesses an apical meristem.
ACKNOWLED GEMENTS
I wish to thank Dr B. Barnes for suggesting this work on Syringa and also Dr H.
Duerden and Dr W. R. Philipson for their kindly help and criticism. I am indebted to
Mr K. 0. Larsen for his technical assistance.
REFERENCE S
BALL, E. ( 1949). The shoot apex and normal plant of Lup·inus alb1ts, bases for experimental morphology.
Amer . .J. Bot. 36, 440.
BaKE, N.H. (1941). Zonation in the shoot apices of Trichocerws 8pachianus and Opuntia cylindrica.
Amer. .J. Bot. 28, 656.
BaKE, N.H. (1947). Development of the adult shoot apex and floral initiation in Vinca rosea. A mer.
J. Bot. 34, 433.
ENGARD, C. J. (1944). Organogenesis in Rubu.~. Univ. Hawaii Res. Publs. no. 21.
1.
G. VAUGHAN
Journ. Linn. Soc. Bot. Vol. Llll, Pl. 24
B.L.F.
Stem apex of Syringa
J. G. VAUGHAN
Journ. Linn. Soc. Bot. Vol. Llll, Pl. 25
at~~~~~W~~F~~~~~*ri~--~' -AD.T.
~~~~:~·~~~ ~~~:A-i~.....~~--tr---...1 - PRO .
.~....,._,~__:..~~T~- VAC.
Stem apex of Syringa
J. G. VAUGHAN: INVESTIGATION INTO STEM APEX OF SYRINGA VULGARIS L.
515
EsAu, K. (1942). Vascular differentiation in the vegetative shoot of Linum. I. The procambiurn.
Amer. J. Bot. 29, 738.
GARRISON, R. (1949). The origin and development of axillary buds. Syringa vulgaris L. Amer. J. Bot.
36, 205.
HELM, J. ( 1931). Untersuchungen iiber die Differenzierung der Sprosscheit elmeristeme von Dikotylen
unter besonderer Berucksichtigung des Prokambiums. Planta, 15, 105.
KAPLAN, R. (1937). tfber die Bildung der Stele aus dem Urmeristem der Pteridophyta und Spermophyten. Planta, 27, :!24.
Loms, J. (1935). L'ontogenese du systeme conducteur dans Ia pousse feuille des dicotylees et des
gymnospermes. La Cellule, 94, 87.
LAwAr"nEE, A. ( l!J48). Histogenese floralo et vegetative ches quelques composees. La Cellule, 52, 213.
MAJUMDAH, G. P. (1942). The organization of the shoot in Heracleum in the light of development.
Ann. Bot., Land., N.S. 6, 4\J.
MAJUMDAH, G. P. (11J47). Growth unit or the phyton in Dicotyledons, with special reference to
Heracleum. Bull. Bot. Soc. Bengal, 1, 61.
~tiLLER, H. A. & W~<;'rMoHI~, H. H. ( Ul4!i). Studies in the developmental anatomy of Phlox drummondii.
Ill. The apices of the mature plant. A mer. J. Bot. 33, I.
PHILIPSON, W. R. (1947). Studies in the development of tho inflorescence. III. The thyrse of Valeriana
officina/is L. Ann. Bot., Lond., N.S. 11, 40\J.
PHILIPSON, \V. R. (IIJ4!J). Tho ontogeny of the shoot apex in Dicotyledons. Biol. Rev. 24, :!1.
l'HIES'l'Lgy, J. H., ScoTT, L. l. & GrLLET'r, E. C. ( l !135). The development of the shoot in Alstroerneria
and the unit of shoot growth in Monocotyledons. Ann. Bot., Land., 49, Hil.
Himv~<;, R H. (l!J42). Shoot apex of Oarrya ell-iptica. Amer. J. Bot. 29, 6!17.
RggvE, lt. H. (19M!). The tunica-corpus concept and development of shoot apices in certain Dicotyledons. Amer. J. Bot. 35, 65.
SIF'I'ON, H. B. (1944). Developmental morphology of vascular plants. New Phytol. 43, 87.
EXPLANATION OF PLATES 24 AND 25
(All photographs from untouched negatives.)
PLATE 24
Fig. l. Photograph ( x 336) of a median longitudinal section, Stt thick, of stem tip of Syringa.
B.L.F., bases of leaf primordia; '1'., tunica; C., corpus; C.Z., central zone; FL.M., flank meristern;
FI.M., tilo meristem; I.Z., intermediate zone.
Fig. 2. Photograph ( x 336) of an oblique longitudinal section showing very young leaf primordium.
L.P. and P.Ji'., leaf primordium and foliar foundation; P.S., procambium strand; A-B, see text;
C-D, see text.
PLATE 25
Fig. 3. Photograph ( x 336) of median longitudinal section showing older leaf primordium.
PRO., procambium; A.M. of L., apical meristem of leaf; AD.T., adaxial tissue; AB.T., abaxial
tissue; VAC., vacuolation; AX.B.Ji'., axillary bud primordium.
Fig. 4. Photograph ( x 336) of oblique longitudinal section showing still older leaf primordium.
P.S., procambium strand; L. of V., limit of vacuolation; S.L., surface layer of cells of leaf
primordium with dense protoplasmic contents.