596 Duthie. — Anatomy of Gnetum africanum. 
and the likeness to companion cells is then very marked (Fig. 12). At the 
periphery of the bundle lies the crushed mass of protophloem. In tangential 
sections through the phloem of older stems, fusiform rays are often seen. 
These vary much in size, some of them being only one cell wide and from 
one to three cells high ; others are several cells wide and higher in pro- 
portion (Figs. 11, id). These rays are specially numerous near the nodes of 
old stems. The cells are thin-walled, with protoplasmic contents, and are 
packed with minute crystals of calcium oxalate. A few starch grains are 
occasionally present. Intermediate stages were observed between the 
smaller rays and albuminous cells with crystallogenous contents (Fig. 16). 
Pith. The pith consists of elongated cylindrical cells with pitted walls 
which are conspicuously lignified in the region of the vascular ring. In most 
of the older internodes the central thinner-walled cells have been disorganized 
(PI. LVII, Figs. 1, 4), though this is not always the case. The older stems 
have the pith cells packed with rounded starch grains ; these are entirely 
absent from the pith of the younger internodes. In the pith region below 
the growing point, and in the pith of the nodes, large numbers of crystals 
of calcium oxalate were found. Running longitudinally through the pith 
are latex tubes similar to those of the cortex which will be described below. 
In the pith of the node large numbers of branched, lignified, fibrous cells 
occur (Fig. 5). Similar sparingly branched lignified elements are very 
occasionally found in the pith of the internode. 
Primary Medullary Rays. As is well known, the primary medullary 
rays are very conspicuous, and extend from pith to cortex, gradually 
broadening outwards (Fig. 1). Their width varies considerably in different 
internodes. These rays are composed of parenchymatous cells which are 
often radially elongated. Many of the cells are lignified and provided with 
simple pits ; others are packed with calcium oxalate crystals and crystal 
sand. The amount of lignification of the ray varies. I11 young stems 
most of the ray cells are lignified ; all have conspicuous protoplasmic con- 
tents, and some are packed with crystals. Between the phloem masses 
large thin-walled crystallogenous cells are very numerous. In older inter- 
nodes a very large number of thin-walled crystallogenous cells occur, and in 
the wider parts of the ray there are stone cells with lignified, pitted, and 
conspicuously striated walls (Fig. 1). These sclerosed cells are very 
numerous in the ray parenchyma between the masses of bast. Starch was 
found to be absent from the rays of the young internodes, but is present in 
considerable quantities in those of older stems. The crystals in the ray cells 
are usually rhombic in surface view and vary greatly in size (PI. LIX, Fig. 30), 
many of them being extremely minute. They resemble closely those found in 
the crystal sacs of phloem and cortex. In tangential section the rays are often 
seen to be broken up into partial rays by vascular tissue (PI. LVII, Fig. 2). 
Single bridging xylem elements were frequently observed (Figs. 2, 3). In one 
