Bot. J . Linn . Soc., 7fI.45-69.With 6 plates and 1 1 figures
January 1975
.
Morphological studies in Argyreia Lour
(Convolvulaceae)
D. D . PANT. F.L.S.
AND
S. BHATNAGAR
Department of Botany. University of Allahabad. Allahabad.2. 11002. India
Accepted f o r publication October 1974
A detailed anatomical study of two species of Argyreia. viz . A . nervosa Bojer and A . roxburghii
Choisy. has been made . The structure of t h e cotyledonary and other nodes and internodes.
including the developmental anatomy of the dominant axillary and accessory branches. the
course and structure of medullary bundles. and the formation of successive rings of cambia
along with the xylem and phloem. have been worked out . Two hitherto unknown types of
saccate tracheidal and vessel elements are described . In t h e light of this work certain
inaccuracies in observations and interpretations of some earlier workers are also corrected .
CONTENTS
. . . . . . . . . . . .
Introduction
Material and methods
. . . . . . . . .
Observations
. . . . . . . . . . . .
External features . . . . . . . . .
Anatomy
. . . . . . . . . . .
Internode . . . . . . . . .
Vascular bundles . . . . . . .
Node
. . . . . . . . . .
Shoot apex
. . . . . . . .
Origin of axillary buds
. . . . .
Origin and differentiation of procambium
Secondarygrowth
. . . . . .
Vascular elements
. . . . . .
Xylem . . . . . . . .
Phloem
. . . . . . .
Discussion
. . . . . . . . . . . .
Axillary and accessory branches
. . . .
Shoot apex
. . . . . . . . . .
Origin of internal phloem
. . . . . .
Medullary bundles
. . . . . . . .
Nodal anatomy
. . . . . . . . .
Vascular elements
. . . . . . . .
Acknowledgements
. . . . . . . . . .
References
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INTRODUCTION
Interesting anomalies such as medullary bundles and successive rings of
cambia; have-been known for a long time t o occur in various members of the
4s
46
D. D . PANT A N D S. BHATNAGAR
Convolvulaceae. For example medullary bundles have been reported in the
genera Argyreia, Rivea, Erycibe, Neuropeltis, Prevostea, Evolvulus and Hewittia
(Solereder, 1908), and successive rings of cambia are known to occur in
Calonyction, Convolvultw, Erycibe, Hewittia, Ipomoea, Mari,na, Merremia,
Porana, Rivea, Argyreia and others (Metcalfe & Chalk, 1950). Details of the
anomalous anatomy of these plants are scarce, however, and practically nothing
is known about their nodal structure except that the nodes of the family are
mentioned as unilacunar with single tripartite leaf traces (Sinnott, 19 14).
Among the above plants we found the anatomy of Argyreia particularly
interesting, since its stems show great diversity in form and arrangement of the
medullary bundles and in other anatomical features, and the material of two
species A . nervosa and A. roxburghii was readily available. As far as we know, no
important additions have been made lately to our knowledge of its anatomy.
The only exceptions are a brief note on the anatomical features of A. nervosa
(A. speciosa) by Johri (1952) and an account of its stomata1 development by
Pant & Banerji (1965); but even these authors left out the details of its
anatomy and nodal structure. Therefore, about three years ago, we started a
detailed anatomical investigation of the anatomy of this genus to fill in some of
the gaps in our knowledge. Meanwhile, Govil (1972) published a brief note on
the developmental anatomy of A. nervosa. However, the information given
there is not only sketchy but also highly inaccurate and misleading.
MATERIAL A N D METHODS
Seedlings and young and old shoots of Argyreia nervosa were collected
locally and shoots of Argyreia roxburghii were obtained from Dehra Dun. The
shoot apices were fixed in Farmer’s fluid and rest of the parts in F.A.A.
Sections were cut by hand or with a Leitz Rotary Microtome after the material
had been embedded in wax in the usual manner. Since the thick coat of hairs
around the stem seemed to hinder the proper dehydration and embedding of
the material, hairs were first carefully shaved with a sharp razor blade (as a
result, the figures of sections show outlines without hairs and sometimes even
without the surface layer). Sections were usually cut at 8 pm, but thicker stems
were sectioned by sledge microtome a t 20 pm. They were ordinarily stained in
safranin and fast green or in gentian violet and mounted in Canada balsam or
D.P.X. (B.D.H.) mountant. For observing the phloem, sections were stained in
Bismark brown, iodine green and lacmoid (Esau, 1948) and mounted in
laevulose sugar solution. Phloem preparations were made permanent by
dehydrating stained slides in ethyl alcohol grades each having 0.5 g of sodium
bicarbonate (Cheadle et al., 1953). They were subsequently cleared in xylol
and mounted in Canada balsam. Individual elements of xylem and other tissues
were observed by macerating pieces of stems in Jeffrey’s fluid and the macerate
was stained in safranin, dehydrated and mounted in Canada balsam.
OBSERVATIONS
External features
In order t o discuss the internal structure of the nodes of Argyreia it is necessary
to understand some of the external features of its shoots. The plant has long
MORPHOLOGICAL STUDIES IN ARGYREZA
47
internodes with spirally arranged leaves which are generally arranged in a 2/5
phyllotaxy. In the axil of each leaf are borne two buds, one of which is axillary
and the other accessory. As a rule, each axillary bud develops into an axillary
branch, which is sometimes as thick as or even thicker than the parent axis
(Fig. IA). Normally the accessory buds remain dormant, but those that occur
beside axillary buds that mature into inflorescence axes develop into shoots.
A natorny *
In ternode
Hand sections of the stem in A. nervosa show that almost every alternate cell
of the epidermis is the base of a two-celled hair, while the frequency of hairs in
A . roxburghii is lower and they show small warts on their surface (Fig. 1G).
Some axial hairs bear club-shaped heads (Fig. 1F). The outer cortex of the
seedling stem in A. nervosa is collenchymatous (Fig. 1E) but that of subsequent
internodes is chlorenchymatous. The chlorenchyma of older stems is replaced
by a periderm. The parenchymatous cells of the cortex and pith show scattered
articulated, non-anastomosing secretory canals, which are lined by epithelial
cells (Fig. 1C). In the floral axis the number of canals increases suddenly at the
level of the insertion of the sepals, and from there they extend into the sepals
right into the tip. Druses occur abundantly, not only in the parenchymatous
cells of the pith and cortex but also in the parenchymatous elements of xylem
and phloem. A sheath of endodermoid cells (Esau, 1968) is usually distinguishable at the inner boundary of the cortex. Its cells contain abundant starch but
lack Casparian thickenings. However, at some places in older stems the
endodermoid layer may be ill-defined or almost indistinct. Inside the
endodermoid layer are a few layers of cells which could be regarded as those of
a pericycle. The inner cells of the pericycle are parenchymatous but those of
the outermost layers become lignified and fibrous (perivascular fibres). Within
this region is a vascular cylinder which encloses a number of medullary bundles.
Vascular bundles
The vascular bundles of the young stem are either arranged in a
perimedullary ring or scattered in the pith. After the adult stem has undergone
a fair amount of secondary growth the perimedullary ring of bundles is
transformed into a vascular cylinder with continuous rings of secondary xylem
and phloem separated by cambium. On the inner side of the secondary xylem
are seen a number of projecting endarch wedges of primary xylem, which are
associated with internal (intraxylary) phloem groups. The bundles are therefore
bicollateral. Arcs of ill-differentiated cambium are present between the primary
xylem and the internal phloem.
Medullary bundles in A . roxburghii (Plate 2B) usually form a broken ring of
arcs inside the peripheral vascular ring. A few bundles may even lie separate in
this broken ring or on its inner side. Almost all bundles of the inner ring are
bicollateral and inverted, i.e. with the protoxylem facing outwards.
Medullary bundles of mature stems of A . nervosa are scattered in the pith
Descriptions that include references to time or movement should be interpreted as referring to serial
sections.
48
D. D. PANT AND S. BHATNAGAR
Figure 1. A-F, A. nervosa: A, apical region of shoot showing axillary branch (ab) as thick as the
main axis (x:); B, seedling showing seemingly opposite cotyledons (x$); C, L.S. articulated
secretory canal ( ~ 3 0 0 )D,
; T.S. secretory canal ( ~ 3 0 0 )E,
. cells from collenchymatous outer
cortex of seedling stem ( ~ 4 0 0 ) ;F, clubshaped hair from shoot apex (~600).
G, two-celled
warty hair of A . roxburghii ( ~ 1 5 0 ) .
MOKPHOLOGICAL STUDIES I N A R G Y R E I A
Figure 2. Different arrangements of tissues in medullary bundles of A . nervosa: A , phloem
strand; B, phloem strand with two xylem elements and cambium between; C, collateral bundle;
D, bicollateral bundle; E, amphivasal bundle; F, amphicribal bundle with crushed cells around it
(all x300).
4
49
50
D. D. PANT A N D S. BHATNAGAR
MORPHOLOGICAL STUDIES IN ARGYREIA
51
(Plate 2A), but they appear to be arranged in arcs in young branches. Their
structure varies greatly, and different types are often found in successive parts of
a single bundle. The following seven arrangements can be observed in transverse sections (Figs 2 and 3): (1) strands of phloem without xylem (Fig. 2A)
and bundles which are (2) collateral (Fig. 2B, C), (3) bicollateral (Fig. 2D), (4)
amphicribal (Fig. 2F), ( 5 ) amphivasal (Fig. 2E), (6) biamphicribal (Fig. 3C) and
(7) obcollateral (Fig. 3A, B). Arcs of cambium are present in all these types of
medullary bundle. Cambial cells are present on both sides of pure phloem
strands and, due to the activity of this cambium, they usually develop later into
bicollateral or occasionally collateral bundles. As well as the concentric
amphivasal and amphicribal bundles, the collateral and bicollateral medullary
bundles show a tendency to become concentric. In the collateral ones the
cambium progressively extends Iaterally so that it ultimately surrounds the
xylem, which in turn becomes surrounded by a ring of phloem with peripheral
elements which may become crushed. The bundles thus become amphicribal. A
bicollateral bundle turns into a concentric bundle by a similar process where
one of the phloem groups becomes surrounded by a ring of xylem and an outer
ring of phloem, having secondary phloem towards the xylem ring and crushed
primary phloem at the periphery. Such bundles may be described as
biamphicribal. Sometimes obcollateral (inversely orientated) bundles are
formed adjacent to the internal phloem (Fig. 3A). Medullary bundles at the
summit of leaf gap are always endarch but the orientation of other bundles in
the medulla is irregular.
The number of medullary bundles varies from 1 to 40 in different parts of a
plant. Further, these bundles may divide and fuse at different levels of a stem
and, in consequence of this, their number also varies. Moreover, the structure
of one and the same bundle changes along its longitudinal course (Fig. 4).
Individual bundles may thus show only phloem at one level and both xylem
and phloem at another, or phloem on one side of xylem at one level and on
both sides at another, or a bundle which is collateral or bicollateral at some
points may become concentric at another.
As the medullary bundles enter an inflorescence axis, their number gradually
diminishes, so much so that they vanish altogether in the pedicel of a flower,
where the peripheral ring shows discrete bundles.
Node
Near the node an arc of the peripheral vascular ring, usually with three or
five primary xylem points, projects into the cortex. At a higher level the
bulging arc separates from the ring to form the leaf trace, leaving a gap in the
ring. The leaf trace bulge of A . roxburghii (Fig. 5A, B) arises from the
peripheral ring opposite two medullary vascular arcs. Towards each flank of the
Figure 3. A. nervosu: A, portion of peripheral vascular ring with obcollateral bundle to the
inside (the xylem of the obcollateral bundle lies so close to the internal phloem of the
peripheral ring that it may appear to have been formed in continuation); B, obcollateral bundle
with more secondary xylem and phloem and crushed primary phloem of the medullary bundle
as well as that of the peripheral vascular ring; C, biamphicribal bundle ( ~ 1 5 0 ) .ph, Phloem.
52
D. D. PANT AND S. BHATNAGAR
leaf trace each medullary arc cuts out a segment (Fig. 5C). Thereafter both
these segments branch unequally, one branch of each segment enlarging and
filling the leaf gap while their counterparts join the two open ends of the
peripheral ring. The open ends now extend outwards t o form the two traces of
an axillary branch (Fig. 5D). The leaf gap is filled by the two branches arising
from the medullary arcs (Fig. 5E) and both the open ends of the peripheral ring
are likewise joined by two strands (Fig. 5F). During the formation of the leaf
trace bulge in A . newosu (Fig. 6A, B), four to six medullary bundles on the
inside divide, fuse with each other and form an arc. Soon after the departure of
the leaf trace the flank bundles of the inner arc join the broken ends of the
peripheral ring (Fig. 6C-F), as in A . roxburghii, to form the branch traces of the
same node, and the remaining bundles of the inner arc bridge the gap (Fig.
6G-I).
However, the cotyledonary node (Fig. 7A, E) and the first few successive
nodes (Fig. 7F-K), which could be studied only in A . nervosu, were
exceptional. In the node above the cotyledons there are no medullary bundles
and in the subsequent one or two nodes there may be one only. Accordingly
the leaf gaps in these nodes are bridged by the internal phloem of the ring
bundles adjacent to the gap (Fig. 7H). The phloem patch is accompanied by an
arc of cambium.
In the cotyledonary node (Fig. 7A-E), the traces of the two seemingly
opposite cotyledons (Fig. 1B) are inserted at slightly different levels. Four
bundles from the vascular ring in the hypocotyl bulge outwards on one side and
fuse t o form the two cotyledonary traces and later divide once again t o form
two larger and two smaller bundles. Slightly above the insertion level of the
first cotyledonary trace and opposite it, another cotyledonary trace is formed
in the same manner.
The vasculature for a floral node (Fig. 8) is far more complicated than that
of an ordinary node. A whorl of traces to the imbricate sepals separates from
the central axis. All the traces do not bulge outwards at the same level, but
they leave in all 20 gaps in the vascular ring. Each sepal is supplied by five
traces, of which the three median are pure sepal traces (st)but the two laterals
are sepal-petal mixed traces (spt),which trifurcate, the lateral branches entering
adjacent sepals the median one the petal. The five traces in each sepal divide
and redivide to form an arc along the adaxial margin of the sepals. Each petal
trace ( p t ) divides first into two and then into the five bundles, all of which
enters a corolla lobe. It may then divide further. Alternating with the original
five corolla traces, five stamen traces (at) then emerge from the central ring of
vascular bundles, leaving one gap for each trace. When the epipetalous stamens
separate out from the corolla tube single stamen traces supply each stamen.
The remaining bundles of the central ring form the carpel traces and lag behind
the dormant shoot apex below the gynoecium.
Shoot apex
The vegetative shoot apex (Fig. 9, Plate 3C) shows a flat to slightly elevated
apical dome, ranging in diameter from 180-270 pm. The two-layered tunica has
an average diameter of 1 1 pm. The outermost layer usually divides anticlinally
but many glandular hairs, which cover the surface of the dome, are formed by
periclinal and anticlinal divisions. Two or three outer layers of the central
MORPHOLOGICAL STUDIES IN ARGYREIA
Figure 4. A . neruosa, selected transections of a medullary bundle from a series to show changes
in structure at successively higher levels in an internode; A, level of insertion of a branch,
medullary bundle bicollateral; B, the bundle becomes concentric with a phloem patch on one
side; C, the bundle once again becomes bicollateral; D, the bundle divides in two; E, one of the
two strands is about to join an adjacent medullary bundle and the other segment will join the
peripheral ring to form a branch trace; F, once again a bicollateral branch arises from the
medullary bundle (with which one of the segments fused) and comes close to the peripheral
ring at the level of the leaf gap; G, the bicollateral branch joins the peripheral ring and partly
fills the leaf gap (~100).
53
54
D. D. PANT AND S. BHATNAGAR
A
c
Figure 5. A. roxburghii, selected transections from a series at successively higher levels to show
the course of vascular bundles from one internode to another; A, T.S. internode showing
peripheral vascular ring and medullary bundles; B. T.S. showing separation of arc of peripheral
ring with three protoxylem points (the arc lies opposite two extended medullary vascular
bundles (arcs)); C, arc of peripheral ring is dividing to form several leaf trace bundles and the
two opposite medullary arcs have divided on the peripheral side and formed branches joining
the peripheral ring; D, peripheral vascular ring extends to form branch traces and at its extremes
towards the leaf further extends to form accessory branch traces; E, abaxial accessory branch
has separated and shows three leaf primordia, while the two medullary bundles of the parent
axis have shifted into the leaf gap; F, leaf gap completely filled by extension of the medullary
bundles (all x20).
A
Figure 6 . A. nervosu, selected transections from a series at successive higher levels to show t h e
course of vascular bundles from one internode to another: A , section of internode, flattened o n
the side of the insertion of leaf in t h e previous node; B, peripheral ring and stem outline appear
convex almost opposite t h e flat side: C, an arc of the peripheral ring (leaf trace arc) has bulged
further and separated o u t o n one side; D, the leaf trace arc has separated.on both sides and four
medullary bundles have lined up in the leaf gap: E, the leaf trace is resolving into an arc of
bundles, while the flank bundles in the leaf gap have moved towards t h e open ends of t h e
peripheral ring; F, flanks of the peripheral ring have extended outwards as branch traces; G,
branch traces have extended distally and later joined into an arc to form abaxial accessory
branch traces and the medullary bundles which are lining up in the leaf gap are ready t o close it;
H, section at t h e base of next internode with (i) a petiole showing two lateral bundles and a
median arc, (ii) axillary small primordium of accessory branch and its first leaf and (iii) t h e
axillary branch with its peripheral vascular ring (dotted) b u t n o medullary bundles; I, T.S. of
next internode (all x8).
55
56
Figure 7. A. nervosa: A-J, selected transections from a series passing through cotyledonary
node, first and second internodes and shoot apex; A, section through hypocotyl showing ring of
bicollateral vascular bundles; B, two cotyledonary traces have emerged almost simultaneously
from the peripheral ring leaving a gap on either side; C, each of the traces divides once t o form
two smaller traces on the adaxial side; D, the cotyledons separate out; E, first internode of the
plant; F, node showing first leaf and gap in the peripheral ring and a leaf t'race arc; G, section
showing branch and accessory branch traces formed by extensions of open ends of peripheral
ring; H, leaf gap is bridged by a few internal phloem patches; I, second internode; J , in the
second node the leaf trace of the second leaf bulges out and a single medullary bundle is seen as
a phloem patch. K, transection of shoot apex showing 6th leaf almost superimposed over 1st
leaf buttress. (A-E, xl2.5; F-K ~ 2 0 . )
57
st
F
n
Figure 8. A. nervosu Selected transsections of floral bud to show vasculature of the floral
nodes; A, T.S. pedicel, showing a ring of bundles and sepal traces ( s t ) in cortex and their gaps in
the ring; B, floral axis showing sepal-petal mixed traces (spr) between groups of pure sepal
traces; C, bulging outlines of sepal bases and one of the sepal-petal mixed traces with three
branches directed towards two adjacent sepals ( s t ) and a petal (pi);D, three sepals with arcs of
bundles already developed, three petal traces ( p r ) and two mixed traces (spt), are seen bulging
out from the central ring; E, sepal base already separated and others separating, petal traces ( p t )
tangentially stretched before division, and two stamen traces ( a t ) ; F, all five imbricate sepals
already separated, corolla tube with a number of bundles, one filament (f, separate and three
more stamens inside the corolla tube, and gynoecium with four peripheral bundles and a
central one; G, five imbricate sepals with a ring of five valvate corolla lobes each with three
main bundles, a ring of five stamens, and the style with two vascular bundles ( ~ 2 0 ) .
58
D. D. PANT A N D S . BHATNAGAR
Figure 9. A. nervosa. L.S. vegetative shoot apex showing different zones and leaf initiation site.
Outer two layers form tunica ( t , and t 2 ) . Rest of the two or three stratified layers form
subtunica ( s t ) below is the corpus ( c ) . Initiation of a leaf ( l i ) is taking place at the second tunica
layer in the flank meristem (densely dotted). (~300)
mother-cell group are more-or-less stratified, 60% showing anticlinal divisions
and only 40% dividing periclinally. These layers form an intermediate zone
between the tunica and the corpus, inside which there is a zone of irregularly
arranged cells with an average diameter of 9.6 pm. Below this zone, the
centrally placed rib meristem consists of highly vacuolated cells in longitudinal
files, while the cells of the flank meristem have densely granular cytoplasm.
Initiation of a leaf starts below a 25-30 pm high dome in the flank meristem. A
small group of surface cells divides anticlinally, while cells in the sub-surface
layers divide irregularly. As a result a leaf primordium arises in the form of a
lateral buttress on the dome. After a very short plastochron another leaf
buttress is formed on the other side.
Origin of axillary buds
A bud becomes visible in the axil of a five-plastochron-old leaf primordium.
Cells of this bud are irregular except in the single tunica layer at the surface.
Soon this bud differentiates its own meristematic zones. Sometimes an abaxial
accessory bud develops similarly between the axillary branch and the leaf.
Origin and differentiation of procambium
A ring of procambial strands, separated from each other by cells which are
also small but distinct from those of procambium, develops from the inner
Figure 10. Xylem elements of A. nervosa: A, B, fibres; C, fibre tracheid with simple and vague
bordered pits; D, unusual saccate tracheidal element with balloon-like bulges showing
contiguous bordered pits; E, trancheid with crossed pit-pores; F, tracheid showing annular
thickenings joined by obliquely thickened bands; G, vessel with spiral thickenings; H, vessel
element showing bulges and simple pits o n the wall; I, vessel element with simple and bordered
contiguous pits; J, vessel element with scalariform pits; K, broad vessel element with simple pits
and a tail at one end; L, broad vessel element with contiguous hexagonal bordered pits on one
face, simple pits elsewhere; M, tracheidal element with reticulate thickenings; N, wood
parenchyma with simple pits. E, F and M x400; others xl50.
MORPHOLOGICAL STUDIES IN ARGYREIA
D
B
v
I
59
F
60
D. D. PANT AND S. BHATNAGAR
zone of the flank meristem. An unusual feature is the presence of additional
discrete procambial strands formed by the rib meristem. These procambial
strands shift towards the peripheral procambial cylinder and join it after the
formation of one or more nodes (Plate 3A, B).
Procambial cells differentiate into phloem outwardly and basipetally and
xylem inwardly and acropetally. Residual meristematic zones, forming outer
and inner cambia, are present on both sides of the xylem. The internal phloem
develops similarly to the external phloem but later. At first the procambial
strands at the periphery of the pith give rise to internal phloem in patches,
which are originally separated from the primary xylem by procambial cells
which appear somewhat different but are also longitudinally elongated
(Fig. 11A). At a later stage they too differentiate into sieve tube elements and a
cambium, which produces outer secondary phloem. The first phloem patches
are therefore regarded as protophloem and the later ones as metaphloem.
Adjacent to the internal protophloem patches there are often a few more of
them with procambial strands that are separated from those of the ring bundles
by parenchymatous elements. These procambial strands produce xylem to the
outside after differentiating a cambium. As a result the bundles become
obcollateral (Fig. 3A). Later on, they differentiate phloem on their opposite
side and turn into bicollateral bundles (Fig. 3B).
The procambial strands of the rib meristem give rise to medullary bundles.
Activity of this procambium starts late. In T.S. these bundles can be recognized
only after the formation of the sixth leaf buttress. In L.S. the cells appear
longer and thinner than the rest of those of the rib meristem. In a seedling a
single procambial strand in the rib meristem is formed first, just above the
insertion of the first leaf after the cotyledons (Fig. 75). Later, the rib meristem
produces a progressively increasing number of procambial strands in continuation with the first one and, when these give rise to medullary bundles, it
appears as if these bundles are repeatedly branched upwards. In an axillary
branch medullary bundles arise from the peripheral ring (Plate 3D). The young
medullary bundles and internal phloems of the peripheral ring (when they are
only phloem patches) may appear to be ending blindly in the pith, since
phloem develops basipetally in advance of the xylem; but as soon as their
xylem develops (acropetally), they become continuous strands.
Secondary growth
In the bundles which form the peripheral vascular ring, the outer cambium
cells produce xylem inwards and phloem outwards, but the cells of the inner
cambium remain inactive for a long time (Plate 1A). The secondary xylem,
which lies next to the primary xylem on its outer side, may show alternating
rings of wide and narrow elements which look like annual rings. Sooner or
later, the activity of the outer cambium decreases and anomalous secondary
rings of cambia are then formed outside the normal vascular cylinder
(Plate 1B). In this process the cells of the pericycle become secondarily
meristematic and start to form xylem towards the centre and phloem towards
the periphery (Plate 4A). After some time the activity of the first-formed
secondary cambium ceases and a new cambium develops from the parenchyma
cells lying inside the ring of perivascular fibres. The process may be repeated, a
series of successive concentric rings of secondary cambia being thus formed.
MORPHOLOGICAL STUDIES IN A R G Y R E I A
Figure 11. Xylem and phloem elements of A . nevuosa: A, first-differentiated xylem elements
near the shoot apex, with undifferentiated xylem and phloem elements o n the right and a few
elements of inner phloem on the left 1x400); B, sieve tube of t h e primary phloem with simple
; similar sieve
horizontal sieve plates ( x l 5 0 ) ; C, enlarged simple sieve plate in side view ( ~ 6 0 0 )D,
plate in surface view (~1000);
E, R.L.S. secondary phloem showing compound oblique sieve
F, similar sieve plate in T.L.S. ( ~ 3 0 0 ) G,
; similar sieve plate in surface view
plates (~1.50);
( ~ 3 0 0 )H,
; enlarged sieve area of a secondary phloem sieve plate (~1000).
61
62
D. D. PANT A N D S. BHATNAGAR
Later the ring of perivascular fibres breaks up into patches, due to the
substantial secondary growth resulting from the numerous rings of cambium.
The inner cambium of the ring bundles cuts off only phloem cells to the
inside (Plate 4B). After a considerable amount of secondary phloem has been
formed, the adjacent obcollateral bundles are separated from the secondary
phloem only by the crushed internal primary phloem (Fig. 3B). A limited
amount of secondary growth also takes place in the medullary bundles. In the
collateral or bicollateral medullary bundles, the cambium situated next to the
protoxylem is more active and extends laterally on both flanks to form a
complete ring, which makes the bundles concentric. Crushed primary phloem
can be seen in many bundles, either on one side of the bundle or all around it.
Vascular elements
Xylem. The xylem of Argyreiu (Fig. 10) includes not only the usual types of
elements, viz. vessels, tracheids, wood fibres, fibre-tracheids and wood
parenchyma, but also some peculiar laterally saccate tracheids and vessel
elements which have not been reported so far in any other plant. The tracheids
are 0.1-1.34 mm long and 0.017-0.05 mm wide with tapering or flat ends. The
vessels may consist of short or long tracheid-like elements ranging from
0.07-1.2 mm in length and 0.02-0.4 mm in diameter. A vessel element may
have one or two tails at each end. Short and wide elements generally occur in
the secondary xylem formed by the successive secondary cambia. Both ends of
a vessel element have simple perforation plates. Tyloses are also occasionally
present.
Longitudinal walls of saccate tracheids or vessel elements (Fig. 10D, Plate
5A-C) show balloon-like bulges protruding here and there. The walls of the
bulges show rather crowded simple or bordered pits and less-crowded pits
elsewhere.
Annular thickenings of the primary tracheids may be joined by obliquely
thickened bands presenting a transitional stage between annular and helical
thickenings (Fig. 10F). Secondary tracheids as well as vessels show variously
thickened elements, viz. spiral (Fig. lOG), reticulate (Fig. lOM), scalariform
(Fig. lOJ) or pitted. Pits in a single element are simple (Fig. 10H, K), bordered
or both together (Fig. 101, L). Bordered pits are either opposite or alternate.
Alternate pits are generally contiguous, appearing araucarioid (Fig. 1OL).
Crossed pit pores were also observed in the secondary xylem of the seedlings
(Fig. 10E).
The wood fibres (Fig. lOA), which may occasionally be septate, are
1.4-3 mm long and 13.2-26 E.tm in diameter. Their lumen is sometimes very
narrow and the walls are up to 7 pm thick with simple pits. Fibre tracheids
(Fig. 10B) resembling tracheids with simple pits or vaguely bordered pits are
also present.
The wood parenchyma (Fig. 10N) consists of rectangular to isodiametric
cells with simple pits. Cells of the axial parenchyma are formed by transverse
divisions in axial-parenchyma mother-cells, which are several times shorter than
the parent cambium cells. Old xylem parenchyma cells may become lignified.
Some cells contain druses also.
Phloem. Mature phloem elements (Fig. 11; Plates 5 and 6) consist of sieve
tubes, companion cells, bast fibres and parenchyma. Sieve plates are simple in
MORPHOLOGICAL STUDIES I N ARGYREIA
63
the internal phloem and simple to compound-scalariform in the extraxylary
phloem. Simple plates are found horizontally orientated (Fig. 11B, C), while
compound sieve plates are oblique to almost vertical (Fig. 11E-G; Plate 6A-C).
The pores of the sieve areas often show callose deposition (Fig. l l D , H; Plates
5D and 6E), which may become so thick as to block them completely. Sieve
areas with callosed pores may also be present on the lateral walls of the sieve
tube elements, but they are smaller (Plate 5E). however, in some sieve plates
pores are devoid of callose (Plate 6D).
Companion cells are shorter than the adjacent sieve tube elements (Fig. 1 1 E ;
Plate 6A). One to seven companion cells usually occur on one side of each sieve
tube element. Other parenchymatous cells are those of the phloem rays, which
are 1-3 cells wide and 10-20 cells in height and length. Parenchymatous cells are
either smooth-walled or have simple pits or reticulate thickenings.
Lignified fibres are rare in the phloem, but fibre-like elongated parenchymatous cells are present. At the periphery of the primary phloem a ring of
perivascular fibres is present. These fibres are 2-7 mm long and 13.2-19.8 pm
thick with walls 5-6.6 pm thick. Sometimes secondary walls almost fill the
lumen of the cell. These secondary walls appear lamellated in section
(Plate 6F).
DISCUSSION
Axillary and accessory branches
One of the features of A. newosa which has not been recorded previously is
the unusually strong development of its axillary branches, which may become
as thick as, or even thicker than, the main stem. This is said to be a feature of
lianas, although such axillary branches have also been reported in Mirabilis
jalapa, Oxybaphus nyctagineus and 0. viscosus by Pant & Mehra (1963).
Shoot apex
Argyreia shows not only two stratified layers of tunica at the periphery, as
mentioned by Govil (1972), but also the two or three stratified layers inside
these that were observed by Ganguli (1956) and Govil (1971) in it and some
other convolvulaceous plants. However, we cannot agree with these authors in
regarding almost all the stratified peripheral layers as those of the tunica, since
we find that only two of the peripheral stratified layers can be regarded as truly
corresponding with a strictly defined tunica layer, i.e. one in which the cells do
not undergo any periclinal divisions. The divisions in the cells of the remaining
three inner stratified layers are also preponderantly anticlinal but a smaller
number of periclinal walls are also formed. Apparently these three layers form
a kind of intermediate zone between a strictly defined tunica and the corpus.
Gifford (1954) regards such layers as forming a stratified corpus, although the
greater frequency of anticlinal divisions in this zone make the term ‘subtunica
layer’ more appropriate in Argyreia. Divisions in the irregularly arranged mass
of corpus cells are in diverse planes.
Although we have preferred a strict definition of tunica, we must regard the
Outer mnica layer of a plant like Argyreia as exceptional since many of its cells
64
D. D. PANT AND S. BHATNAGAR
show periclinal divisions to give rise to hairs. Even though these are not the
ordinary divisions of the tunica cells, yet their special character is difficult to
recognize till they have formed the hairs.
Origin of internal phloem
There seems to be no unanimity among the previous workers regarding the
origin of the internal phloem in dicotyledonous plants. Peterson (1882) and De
Bary (1884) regarded that the internal phloem of members of Myrtaceae,
Apocynaceae, Asclepiadaceae, Convolvulaceae, etc. as procambial in origin and
part of the vascular bundle. Herail (1885) said that this was true only for the
Cucurbitaceae, but that in other families the internal phloem arose from the
pith cells. Lamounette (1890), Lee (1912) and Hayward (1932) doubted the
procambial origin of the internal phloem in Convolvulus and Ipomoea,
believing that it arose from the parenchyma which is adjacent to but sharply
delimited from the procambium and thus arises independently of the vascuIar
tissue. Baranetzky (1900), who made extensive studies of the internal phloem,
suggested that in Operculina tuberosa and Convolvulus arvensis the procambium was more sharply delimited on the outer margin than on the inner
side and that it continued to widen even after the differentiation of the first
protoxylem elements, the new cells developing into internal phloem. Fukuda
(1967) reached the same conclusion on the basis of histogenetical studies of the
derivation of internal phloem in six species of Convolvulaceae (Calystegia
hederacea, C. nipponica, C soldanella, Ipomoea batatas var. edulis, I.
pes-carpae and Pharbitis congesta). According to him, when the protoxylem
differentiates, a few cells just inside it are more or less clearly distinguished by
their procambial nature from the more internal medullary cells. Our observations about the procambial origin of the internal phloem in Argyreia are in
agreement with previous observations made by Peterson, De Bary, Baranetzky
and Fukuda about the similar origin of internaI phloem in other members of
the Convolvulaceae. This would suggest that the vascular bundles of the normal
ring in Argyreia and other convolvulaceous plants could be called bicollateral as
defined by HCrail.
Medullary bundles
The medullary bundles in fully grown internodes of A. nervosa are scattered
in the pith, but they tend to be arranged in a loose ring of broken arcs in the
internodes of branches which are not fully elongated. At this stage their
arrangement resembles that of A. roxburghii. I t would appear, therefore, that
the arrangement of medullary bundles in the two species of Argyreia is
basically similar.
According to Govil (1972), in A . nervosa there are only eight to ten
medullary bundles which may be bicollateral, concentric, semiconcentric or
obcollateral. Our observations show that the number of bundles in this plant
can be far greater and also greatly variable, since they frequently divide or
anastomose. At certain levels their number may increase to 40 or even more.
Again, all the medullary bundles are developmentally similar and not
classifiable into the different categories mentioned by Johri and Govil. Actually
MORPHOLOGICAL STUDIES IN ARGYREIA
65
these different kinds are only bundles of the same nature seen or caught in
different stages of development. At first they all show themselves merely as
small strands of phloem. When the xylem elements develop, these strands
become collateral, bicollateral, amphivasal, amphicribal or obcollateral bundles.
Johri reported that the medullary bundles were usually collateral; but again this
is only due to developmental differences, and we found that in a majority of
our sections the vascular bundles were bicollateral, their phloem always being
better developed on the protoxylem side. Further, they also show the tendency
to become amphicribal. The obcollateral medullary bundles which originate
close to the internal phloem are regarded by Solereder (1898) as arising from it.
However, we found that the obcollateral bundles arise from separate
procambial strands. At first they produce phloem strands, which are situated
close to the strands of internal protophloem of the ring bundles and are
indistinguishable from them, since both kinds of strands appear in T.S. as
phloem islands at the margin of the pith. In the thicker stems, where a good
deal of secondary growth has taken place, the outer elements of the primary
phloem of an obcollateral bundle (which later develops outer phloem also) may
appear crushed against the inner primary elements of the internal phloem in the
vascular ring.
Plants with medullary bundles have been categorized by De Bary (1884)into
three groups: (1) Plants where all bundles in the ring as well as the medullary
ones belong to the leaf traces. This group includes most of the Cucurbitaceae,
and species of Amarantaceae, Phytolaccaceae (Phytolacca dioica), Piperaceae,
Berberidaceae (Diphylleja, Leontice), Papaveraceae (species of Papaver) and
Ranunculaceae (Thalictrum and Actaea). ( 2 ) Plants where all bundles belong to
the leaf traces but in their downward course into the stem form a network of
bundles which branch irregularly on all sides so as to appear scattered like those
of a monocotyledon. To this group belong the Nymphaeaceae, Gunneraceae,
Primulaceae (Primula uuriculu) and its nearest allies and many Balanophoraceae. ( 3 ) Plants where both cauline and leaf trace bundles are present, the
leaf trace bundles belonging to the ring while the medullary bundles are
cauline. To this group belong Bignoniaceae, Orobanchaceae, Cactaceae (species
of Mumillaria), Nelumbium, Melastomataceae and some Umbelliferae and
Araliaceae. According t o Govil’s (1972) description, Argyreia would seem to
belong to the third category. However, we have carefully traced the
course of the medullary bundles vis ci vis the bundles of the ring and
leaf traces and find that the description given by Govil does not take into
consideration the course of medullary bundles in successive nodes and
internodes. Actually the medullary bundles in this plant at first move to the
ring, filling the leaf gaps, and thereafter continue to remain in the peripheral
cylinder throughout a few internodes and nodes, ultimately passing out as leaf
traces. Therefore Argyreia cannot fit in precisely in any one of De Bary’s three
categories. Indeed these categories may themselves require to be reconsidered.
As early as 1883, Weiss had described the formation of leaf traces and the
origin and course of medullary bundles in the internodes of some members of
the Compositae, and the condition there appears to be similar to what we have
presently found in Argyreia. Shortly after the origin of a branch, the bundles
which are t o become medullary leave the vascular ring and rejoin it at a higher
level in the node, closing the leaf gap. However, medullary bundles of other
5
66
D. D. PANT AND S. BHATNACAR
members of Compositae, described by Davis (1961), differ from those of
Argyreia in forming a nodal plexus with branches which accompany leaf traces
and also supply the axillary branches without deflecting into the peripheral
ring.
According to Govil (1972) the medullary bundles of Argyreia nervosu “do
not have continuity from base to apex and also they do not join the bundles of
the other branches.” If the above statement is correct we ought to have
observed blind ends of medullary bundles here and there, but we failed to see
any of these (except when the bundles were immature). Instead we found that
the medullary bundles join the peripheral ring at the summit of each leaf gap to
form the branch traces of the same node. In this respect the formation of
branch traces in Argyreia resembles the condition in Nyctaginaceae (Pant &
Mehra, 1961, 1963) and Amaranthaceae (Inouye, 1956). However, their
further course differs; in the branches of Argyreia they partake in the
formation of the peripheral ring, whereas in members of Amaranthaceae and
Nyctaginaceae they are connected only with the so-called medullary bundles of
the branch and the peripheral ring (belated ring) surrounds them.
Worsdell (1919) suggested that the presence of medullary bundles in an
angiosperm is a vestige of its monocotyledonous ancestry; but their absence in
admittedly conservative parts like the cotyledonary node and the pedicel may
suggest that they are a later acquisition, a t least in the presently investigated
plants. Moreover, the form and structure of medullary bundles themselves
present no similarities to typical monocotyledonous bundles and appear rather
to be an advanced feature, an adaptation for a climbing habit.
Nodul anatomy
The unilacunar nature of the nodes of Argyreia is in agreement with the
observations of Sinnott (1914) on the nodes of Convolvulaceae. Applying the
theory of recapitulation, the unilacunar two-trace condition of the cotyledonary nodes, the tripartite foliar strands in a few succeeding nodes and the
pentapartite foliar strands in all adult nodes may favour the idea of Canright
(1955) and Bailey (1956) that unilacunar two-trace nodes are primitive.
However, in the absence of data about cotyledonary nodes for a large number
of angiosperms and more particularly for the primitive ones, it is not possible
to make any categorical statement in this connection.
Vascular elements
At first we suspected that the balloon-like bulges of the newly observed
saccate tracheid and vessel elements might represent elements of adjacent
tracheid or vessels sticking to another thick-walled cell; but fine focussing
clearly showed that the lumen of the tracheary element and the balloon-like
outgrowths was continuous. Their mode of formation is unknown. Their
frequent and invariable occurrence in stems of both species of Argyreia
indicates that they are no abnormality. The function of these elements is not
understood, but they may possibly be acting as water reservoirs in the
excessively elongated stems of a climbing plant under conditions of water
scarcity.
MORPHOLOGICAL STUDIES IN ARGYREIA
61
Although the stages in the development of the association of multiple
companion cells with each tube element could not be followed, it is assumed
that they are formed in the normal manner, that is by the formation of one or
more transverse partitions in the primary companion cell.
The secondary xylem may be regarded as of an advanced type since it shows
(i) rather short vessel elements, (ii) usually simple pits on the longitudinal walls,
(iii) invariably simple perforation plates and (iv) vertically as well as
transversely seriate parenchyma cells. Such highly advanced xylem is, however,
accompanied by a very primitive type of secondary phloem (Esau et aL, 1953),
since it shows (i) compound sieve plates on the end walls, (ii) oblique or
sometimes almost vertical orientation of the sieve plates and (iii) the presence
of sieve areas on the side walls. But the sieve tubes of both the intra- and
extra-xylary primary phloem are rather of an advanced type, having simple
horizontal sieve plates and no sieve areas on the longitudinal walls. Indeed the
above observations may indicate that the trend of specialization in phloem has
been the reverse of that observed by Frost (1930a, b, 1931) in xylem.
Accordingly, while an advanced type of sieve tube has developed in the primary
phloem of Argyreia, its secondary phloem has retained the primitive type of
sieve tube elements with oblique scalariform sieve plates and sieve areas on the
longitudinal walls. Sp-eaking ontogenetically it may seem that, while the xylem
evolved by becoming highly specialized, the specialization in the phloem was
reversed, so that the secondary phloem elements may provide an instance of
the phenomenon of evolution through “Paedomorphosis” (see Gaussen, 1942 ;.
De Ferrt, 1952; De Beer & Swinton, 1958). The retention of advanced
characters by the primary phloem would, however, be an unusual feature.
ACKNOWLEDGEMENTS
I t is a pleasure to record our thanks to Dr M. Anantswamy Rau, Deputy
Director, B.S.I., Dehra Dun, Miss Giribala Pant and Professor T. S. Mahabale,
Poona for supplying material of A . roxburghii and A . newosa.
REFERENCES
Papers marked with an asterisk ( * ) have not been seen in the original.
BAILEY, I. W., 1956. Nodal anatomy in retrospect. J. Arnold Arbor., 37: 269-87.
OBARANETZKY, J., 1 9 0 0 Recherches sur les faisceux bicoIIatCraux. Annls Sci. nut. Bot. (shr. 8 ) . 12:
261- 3 32.
CANRIGHT, J. E., 1955. The comparative morphology and relationships of the Magnoliaceae. IV. Wood
and nodal anatomy. J. Arnold Arbor., 36: 119-40.
CHEADLE, V. I., GIFFORD, E. M., Jr. & ESAU, K . , 1953. A staining combination for phloem and
contiguous tissues. Stain TechnoL, 28: 49-53.
DAVIS, E. L., 1961. Medullary bundles in the genus Dahlia and their possible origin. A m . J. Bot.. 48:
108-13.
DE BARY, A., 1884. Comparative anatomy of the vegetative organs of the Phanerogams and Ferns
(Transl. F. 0. Bower & D. H. Scott). Oxford.
DE BEER, G. & SWINTON, W. E., 1958. Prophetic fossils. In T. S. Westall (Ed.), Studies offossir
vertebrates: 1 - 1 5 London: Univ. of London.
DE FERRE, Y., 1952. Les forme de jeunesse des AbiCtacCes: ontoghiephyloghie. Tmv. Lab. for.
Toulouse, t, 2, 3: 1-282.
ESAU, K., 1948. Phloem structure in the Grapevine and its seasonal changes. Hilgardia, 18: 217-96.
ESAU, K.,1968. P&nt anatomy. New York: Wiley.
ESAU, K., CHEADLE, V. I. 84 GIFFORD, Jr. E. M., 1953. Comparative structure and possible trends of
specialization of the phloem. A m . J. Bot., 40: 9-19.
D. D. PANT AND S. BHATNAGAR
68
FROST, F. H., 1930a. Specialization in secondary xylem of dicotyledons. 1. Origin of vessel. Eot. Gar.,
89: 67-94.
FROST, F. H., 1930b. Specialization in secondary xylem of dicotyledons. 11. Evolution of end wall of
vessel segment. Bot. Gaz., 90: 198-212.
FROST, F. H., 1931. Specialization in secondary xylem of dicotyledons. 111. Specialization of lateral wall
of vessel segment. Eor. Gar., 91: 88-96.
FUKUDA, Y.,1967. Anatomical studies of internal phloem in the stems of dicotyledons, with special
reference to its histogenesis. J. Fac. Sci. zlniv. Tokyo, Sect. N I (Eot.), 9: 313-75.
GANGULI, B., 1956. Shoot apex organization in Convolvulaceae. Proc. 43rd Indian Sci. Congr., 3: 242.
GAUSSEN, H., 1942. Evolution e t retour a w formes ancestrales. Revue gkn Sci pur. appl., 52: 4.
GIFFORD, E. M., Jr., 1954. The shoot apex in angiosperms. Bot. Rev., 20: 477-529.
GOVIL, C. M., 1971. Morphological studies in the family Convolvulaceae. 111. Zonation in the shoot apex
and leaf initiation. J. Indian bot. Soc.. 50: 217-225.
GOVIL, C. M., 1972. Developmental studies in Argyreia nervosa Boj., 110-19. In V. Puri et al. (Eds),
Symposium on biology of the tand plants. Meerut: Meerut University.
HAYWARD, H. E., 1932. The seedling anatomy of Ipomoea batatas. Eot. Gar.. 93: 40020.
*HERAIL, J., 1885. Recherches dur I’anatomie comparie de la tige des dicotyledons. Ann. Sci. Nut..
Eot., sir. 7, 2: 203-314.
INOUYE, R., 1956. Anatomical studies on the vascular system of Mirabilisjalapa L. E o f . Mag. Tokyo, 69:
554-59.:
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Congr. (Calcutta): 28.
*LAMOUNETTE, R., 1890. Recherches 5 1’Ctude du liber des angiospermes. Ann. Sci. nar. Eot. (sCr. 7/,
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LEE, E., 191 2. Observations o n the seedling anatomy of certain sympetalae. 1. Tubiflorae. Ann. Eot., 26:
727-46.
METCALFE, C. R. & CHALK, L., 1950. Anatomy of the dicotyledons. Oxford: Clarendon Press.
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Convolvulaceae. Senckenberg. b i d . 46: 155-173.
PANT, D. D. & MEHRA, B., 1961. Nodal anatomy of Eoerhaavia diffusa L. Phytomorphology, 1 1 :
384-4 05.
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India, 29: 41-76.
PETERSON, 0 . G., 1882. Uber das Auftreten bicollateraler Gefassbundel in verschiedenen Pflanzenfamilien und uber den Werth der selben fur die Systematik. Eot. Jb., 3: 359-402.
SINNOTT, E. W., 1914. Investigation o n the phylogeny of the angiosperms. I. The anatomy of t h e node
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EXPLANATION O F PLATES
PLATE 1
A . nervosa
A. T.S. of stem with portion of peripheral ring, showing prominent internal phloem patches
(~180).
B. T.S. of old stem showing several anomalous secondary growth increments (x9).
PLATE 2
A. A . nervosa T.S. of stem showing scattered medullary bundles ( ~ 2 0 ) .
B. A . roxburghii. T.S. of stem showing arcs of inverted medullary bundles and a few discrete
ones tending t o form a ring inside the peripheral ring ( ~ 3 5 ) .
Bot. J. Linn. Soc., 70 (1975)
I). D. P A N T
AND
S. BHATNAGAK
Plate 1
(Facing p . 68)
Bot. J . Linn. SOC.,70 (1975)
I ) . 1). P A N T
AND
S. BHA'I'NAGAII
Plate 2
Bot.
r. Linn. Soc., 70 (197.5)
I). I). PAN'I'
AS[)
S. UHATNAGAR
Plate 3
Bot. J. Linn. Soc., 70 (1975)
D. D. PANT
AND
S. BHATNAGAR
Plate 4
Bot. 1. Liniz. SOL.,70 (1975)
I). D. P A N T
AND
S. BHATNAGAR
Plate 5
Rot. J. Linn. SOC.,70 (1975)
I). D. PANT'
AND
S. RHATNAGAR
Plate 6
MORPHOLOGICAL STUDIES IN ARCYREIA
PLATE 3
A. nervosa
A. B. L.S. stem apex showing connections of medullary bundles with the peripheral ring in the
region of a node (~50).
C. Shoot apex showing stratified tunica and subtunica layers and distinct flank and the rib
meristem ( ~ 1 7 5 2
D. L.S. stem a t nodal level showing connection of medullary bundle of axillary branch with
peripheral vascular ring of the parent axis (~10).
PLATE 4
A . nervosa
A. Portion of stem
T.S.showing perivascular fibres in the form of dark patches and a bundle of
the youngest ring showing cambiform cells in between xylem and phloem ( ~ 1 8 0 ) .
B. Portion of peripheral vascular ring from stem T.S. showing internal phloem with crushed
primary phloem ( ~ 1 0 0 ) .
PLATE 5
A. Saccate tracheidal element with lateral balloon-like bulges (~135).
B. Three balloon-like bulges of same tracheidal element to show bordered pits over the bulges
and simple pits elsewhere ( ~ 5 0 0 ) .
C. Bulges of another tracheidal element (~500).
D. Sieve plate of a primary sieve tube element having ring-like callose deposition around pores
(x22SO).
E. Sieve areas o n the lateral walls of secondary sieve tube elements ( ~ 6 0 0 ) .
PLATE 6
A. R.L.S. of secondary phloem showing sieve tube elements with surface views of end walls of
compound sieve plates (sieve areas are seen as dark patches) (xl50).
8. Single compound sieve plate enlarged ( ~ 6 0 0 ) .
C. Compound sieve plate in section ( ~ 6 0 0 ) .
D. Surface view of sieve plate where sieve areas appear lighter due t o lack of callose deposition
(~500).
E. Sieve areas of compound sieve plates; dark areas are callose cylinders each lining a pore
(~2250).
I:. Perivascular fibres showing lamellations ( ~ 3 6 0 ) .
69