Duthie . — Anatomy of Gnetum africanum . 597 
section a large pitted vessel was seen bridging a ray, and in a few cases 
bridging phloem elements were also noted. 
Cortex. The ground tissue lying next to the phloem masses consists, 
in young stems, of thin-walled parenchymatous cells through which a few 
latex tubes and fibres are scattered. In older stems an irregular ring of large 
parenchymatous cells becomes strongly lignified, forming the conspicuous 
sclerenchymatous zone (Fig. 1 and PI. LVIII, Fig. 9). The walls of these stone 
cells are stratified and pitted, and resemble the sclerotic elements of the old 
medullary rays. The fibres are elongated and unbranched, with cellulose 
walls and pointed ends. They are very numerous in the outer cortex, but are 
occasionally found inside the zone of stone cells. The white lustrous thicken- 
ing layer is often found to have separated from the primary wall (PI. LIX, 
Fig. 27). The marked contraction which sometimes occurs may have been 
caused by the alcohol in which the material was preserved. The cell lumina 
of many of the fibres are almost entirely obliterated ; the walls of some show 
well-marked stratification (Fig. 27). In the cortex of the node, stellate 
fibrous cells also occur (PI. LVII, Fig. 5), but they are extremely rare in the 
internode. In the younger internodes the parenchyma of the cortex, especially 
near the epidermis, contains numerous chloroplasts. A considerable quantity 
of storage starch is present in the cortex of older stems, and is very abun- 
dant in the parenchyma between the zone of stone cells and the phloem 
masses. Crystallogenous cells, similar to those of the rays, also occur 
in the cortex. 
Latex Tubes. Among the most striking elements of the cortex are 
unbranched secretory tubes similar to those found in the pith and leaf-stalk. 
These laticiferous vessels of Gnetum africanum do not differ from those 
of Gnetum Gnemon described by Bower. 1 Their origin from vertical rows 
of elongated parenchymatous cells was observed in microtome sections 
of the young stem below the apical cone. Two or three nuclei were 
observed in several of these cells. Occasionally short, lateral protrusions of 
these vertical tubes were found forcing themselves between the neighbour- 
ing cells. The coagulated, albuminous contents turn a golden yellow on 
treatment with iodine. On placing longitudinal sections of the stem in 
strong sulphuric acid, the cellulose walls are dissolved and the strands of 
coagulated latex show up very distinctly. After prolonged treatment with 
alkanet tincture the contents of the tubes turn a reddish brown. 
Epidermis of Stem. The epidermal cells of the young internodes are 
papillate, with much thickened outer wall and well-defined cuticle. The 
wall thickening in older internodes often projects irregularly into the cell 
cavity. The stomata are arranged in vertical rows and sunk below the 
level of the epidermis (PI. LIX, Fig. 23). Their structure resembles that 
of the leaf stomata which will be described below. 
1 Bower (2). 
