164 Gibson . — Contributions towards a Knowledge 
runs down the stele below as a dorsal cord, which does 
not fuse with the marginal protoxylem of the other side until 
after the origin of the next older branch, when that marginal 
protoxylem has in turn become a dorsal cord (PL IX, Fig. 21). 
The section of the stem is oval, and the stele is excentric in 
position, being nearer the ventral side. There is a cuticularized 
epidermis, and two to three layers of stereome. The cortex is 
thin-walled, and has a siliceous deposit on the inner cells. 
The trabeculae consist as usual of an endodermal cell and 
a cluster of parenchymatous cells containing chlorophyll next 
the cortex. The pericycle is as a rule one layer thick, though 
here and there two layers occur. The crushed protophloem- 
elements are numerous. The sieve-tubes are two layers deep, 
though occasionally reduced to one layer, and are quite absent 
opposite the marginal protoxylems. The metaxylem is late 
in development, and frequently shows triradiate elements at 
the point of origin of branches. There are, as a rule, two 
layers of phloem-parenchyma. 
Dangeard (22) gives no description of the course of the 
protoxylems, but figures an arrangement which I have not 
been able to confirm. 
28. Selaginella producta , Baker. Baker’s Handbook, No. 87. 
The arrangement of the protoxylems in this species is on 
the plan of that seen in X. atroviridis. The loose trabecular 
tissue forms a reticulum somewhat like that figured for 
S. Braunii (PI. IX, Fig. 27). The cells composing the reticulum 
are frequently strongly cuticularized throughout their entire 
length. There is a well-marked siliceous deposit on the 
inner cortex and round the distal trabecular cells. The inner 
cortical cells are tubular, like those of X. grandis. The peri- 
cycle in full-grown stems is about three layers thick. The 
sieve- tubes are large, and two or occasionally three layers 
deep dorsally and ventrally, and one-layered opposite the 
marginal protoxylems. This layer is often interrupted in 
that region in smaller and younger stems. The phloem- 
parenchyma is one to three layers deep. 
29. Selaginella bisulcata , Spr. Baker’s Handbook, No. 273. 
