600 Duthie.— Anatomy of Gnetum africanum. 
The epidermis is provided with a well-marked cuticle which shows up 
distinctly on treatment with alkanet root or a fresh solution of chlorophyll. 
The stomata are small, numerous, and arranged irregularly over the 
whole of the under surface with the exception of the veins and veinlets. 
A few are also present on the upper surface over the midrib. 1 They occur 
on the scale leaves as well as on the foliage leaves. Each stoma is accom- 
panied by two subsidiary cells lying parallel to the pore (Fig. 22), but the 
material did not admit of the determination of the origin of the subsidiary 
cells. Several cases of ‘twin stomata’ were observed. In the leaf the 
stomata are about on a level with the subsidiary cells (PI. LIX, Fig. 24), but 
in the young stems the stomata, which are here placed parallel to the longi- 
tudinal axis, are considerably sunken (Fig. 23). Seen from above the stoma 
often bears a striking resemblance to the under surface of a cowrie shell 
(PI. LVIII, Fig. 22). The outer wall of each guard cell is provided with a 
lignified flap marked by delicate striations (Fig. 22). The guard cells, as seen 
in longitudinal section, are enlarged at the ends and somewhat contracted in 
the middle (PL LIX, Fig. 25). They contain large quantities of starch. As 
in most investigated Gymnosperms, ridge-like protuberances of the membrane 
are absent. 
Summary. 
Gnetum africanum is heterophyllous, producing pairs of scale leaves 
which subtend the lateral branches, and foliage leaves with abortive buds 
in their axils. The stems examined showed normal secondary thickening. 
The number of vascular bundles seen in transverse section of the axis 
varies considerably. 
The first-formed vessels have several perforations on their sloping end 
walls, and transitional stages were found between these and the later-formed 
vessels with a single circular perforation. As is usual in climbers, the 
vessels of the secondary wood are of considerable size. 
The phloem tissue is composed of regular rows of sieve tubes alternating 
with rows of elongated albuminous cells. Numerous compound sieve plates 
occur on the oblique end walls and also on the lateral walls of the sieve 
tubes. Considerable callus formation takes place. 
In addition to the conspicuous primary medullary rays, smaller fusiform 
rays packed with calcium oxalate crystals are sometimes found. Tran- 
sitional forms between the latter and the elongated albuminous cells of the 
phloem occur. Rhombic crystals of calcium oxalate, which vary greatly in 
size, are present in large numbers in certain cells of the xylem, phloem, 
medullary rays, cortex, and phelloderma, also in the pith of the nodes. 
Sclerenchymatous elements of various forms occur in the stem. Stone 
cells with lignified and pitted walls form a conspicuous zone outside the 
1 Griffiths (5). 
