petiole and root as due to the growth in length which ruptures or pulls together the soft walls of the annular vessels. However, Miller (1952) believes that the cells differentiate as metaxylem rather than proto- xylem because maturation is delayed until elongation of the tissue has stopped.
The remainder of the vascular tissue (after departure of the cotyledonary traces) supplies buds and stem of Allionia as in Boerhavia
(Nair and Nair, 1961) . The non-vascular anatomy of the root and hypo- cotyl of Allionia is also similar to Boerhavia (Nair and Nair, 1961).
The Adult Plant
The Vegetative Shoot Apex
Following the terminology of Schmidt (1924), the apical meristem of Allionia has a biseriate tunica, whose cells undergo anticlinal
divisions to increase the surface growth and form the protoderm. An occasional periclinal division occurs in the second layer. In the
corpus beneath, cells divide periclinally and obliquely as well as anti- clinally to increase the volume of the apex. Small, densely cytoplasmic cells in the peripheral zone contribute cells to the ground meristem which provided the cortex of the stem and procambial tissues of the leaves. The rib meristem, originating from the base of the corpus, pro
vides the pith of the stem by an increase in the cell number and then through growth and expansion of the derivative cells.
Leaf initiation. The opposite leaves are initiated 30-40 pm below the apex by periclinal divisions in the peripheral zone of the apical meristem, followed by anticlinal divisions in the tunica. Divi
sions are propagated circumferentially around the apex to form the large leaf bases characteristic of Allionia.
The young leaf primordia are of slightly unequal height from an early stage of development but are of approximately equal thickness and width. A conspicuous adaxial meristem contributes to growth in thick
ness by periclinal divisions, much of the tissue to which it gives rise later differentiating as collenchyma. One to three procambial strands are present in the developing leaf primordia.
The lamina of the leaf is formed by marginal meristems which develop when the leaves are approximately 200 pm long. The panels of tissue formed turn toward the shoot axis. The protoderm ,divides anti- clinally to produce the epidermis of the leaf. The submarginal initials contribute cells to the interior tissues of the blade according to the pattern shown'in fig. 13„
Veins develop from the middle layers of the plate meristem, primary veins first, followed by the secondary and tertiary veins. Few intercellular spaces are formed in the palisade layers of the lamina;
much larger intercellular spaces are formed in the spongy layer during the later stages of leaf maturation.
Bud initiation. The inception of axillary buds can first be observed in axils of late plastochrone (second youngest pair). The
60
a d
pr
m i
s p
a b
Figure 13. Growth of leaf lamina.
ab, abaxial epidermis; ad, adaxial epidermis; mi, marginal initials;
p, palisade layer; pr, procambium; si, submarginal initials; sp, spongy parenchyma; v, vascular bundles.
first sign of bud formation is development of the shell zone — anti
clinal divisions delimiting the bud meristem from the apical meristemu Bud primordia formed in the axils of a pair of opposite leaves are of unequal size from their inception (fig. 14). The initial dif
ference in size between bud primordia of a single node becomes enhanced during subsequent growth. Usually the smaller bud does not develop be
yond a meristematic dome for a long period of time. From the time the small bud is emergent it lacks the shell zone.
The cells delimited by the shell zone undergo active division in various planes to give rise to the axillary bud meristem, which is evi
dent as a densely staining region of meristematic cells. Three layers of the apical meristem take part in the formation of the bud two layers of the tunica and the outer layer of the corpus. The shell zone cells contribute the procambium and pith of the bud plus the cortical ground meristem of the axis. It is present until initiation and early development of the leaf buttresses on the large bud.
During later stages of development, the bud meristem becomes organized in a manner similar to the apical meristem and gives rise to leaf primordia. As noted previously, once the original apical meristem of the plant develops into a reproductive apex each axillary bud pro
duces one set of leaves (each with its axillary buds) and terminates in a reproductive apex (fig. 14).
The Leaf
Histology of the petiole. The petiole (fig. 15) of Allionia has three veins: one lateral vein on either side and slightly adaxial to
62
Figure 14. Longitudinal section of bud initiation.
X190. Note node with terminal inflorescence in center, large stem on left, and small stem on right, fa, floral apex; Ivb, large vegetative bud; svb, small vegetative bud.
>1-5 6 < V
x.
Figure 15. Transections of petiole of leaf.
A. Petiole midway between stem and lamina. X120. Note large lateral veins and less prominent midvein. B. Pulvinus. X360.
the less prominent midvein» Occasionally more than three veins are present in the petiole,,
The mature petiole has a single layered epidermis of polygonal cells5 with numerous uniseriate glandular trichomes* Stomata are pres
ent but the substomatal chamber is very small« Just beneath the
epidermis is a single layer of collenchyma. The remainder of the cortex is composed of isodiametric chlorenchymao The collateral vascular
bundles are slightly arcuate toward the adaxial surface of the arcuate petiole (in transverse view). Each bundle contains primary phloem ab- axial to the primary xylem* A cambium usually develops between these tissues, and some secondary xylem and phloem is produced. Collenchyma is present around each vascular bundle. Bundles of raphides may occur in chlorenchyma cells abaxial to the vascular bundles.
Just below the base of the blade (fig. 15B) a groove occurs in the.adaxial surface of the petiole above the arc of the veins. The cells of the epidermis are slightly smaller than elsewhere on the pet
iole, and just beneath them a band of collenchyma is present. This region functions as a pulvinus, regulating the opening and closing of the leaf blade.
Histology of the lamina. The heavily cutinized epidermal cells are triangular to polygonal in outline (having three to nine sides) on both the abaxial and adaxial surfaces of the lamina (fig. 16). Func
tioning anomocytic stomata are present on both surfaces of the leaves.
The guard cells are about 24 jum long and are level with the surface of
64