of the Anatomy of the Genus Selaginella , Spr. 191 
As already described, the structure of the rhizome is very 
different from that of the erect axis. The large-celled cortex 
towards the stele becomes smaller-celled and almost, save for 
a few small intercellular spaces, continuous with the outer 
pericycle layers of the cylindrical stele. (Compare the creep- 
ing axis of 5 . inaequalifolia , p. 181.) In these spaces occur 
cells which are homologous with the endodermal cells, but 
whose walls do not show the usual cuticular band. On the 
other hand, their walls are wholly cuticularized and they stand 
out quite prominently in sections as bright clear rings. There 
follows, passing inwards, several layers of small-celled peri- 
cycle, and then an interrupted layer of small tangentially 
flattened sieve-tubes. The sieve-tubes are separated from 
the xylem by (on an average) two layers of phloem-paren- 
chyma, quite similar to and, in section, about the same 
size as the cells of the pericycle. The ring of xylem is 
quite complete (save where, as already explained, the erect 
shoots arise) and is on an average two tracheides broad. On 
the outer side and alternating with the patches of small 
sieve-tubes lie four, or near the origin of erect shoots, several 
protoxylem-areas. Within the tracheidal ring one meets with 
another double layer of phloem-parenchyma, followed by one 
or usually two layers of large sieve-tubes, continuous round 
the inner side of the stele. These are followed by a pericycle 
similar to that on the outside and by endodermal cells wholly 
cuticularized and lying in an irregular and not well-marked 
lacuna and connecting the pericyles of the inner and the outer 
steles. The structure of the inner stele is somewhat different. 
There is a pericycle about two layers deep and a single, 
or occasionally double, layer of sieve-tubes completely en- 
closing a single layer of parenchyma which in turn surrounds 
a central mass of xylem consisting of about a dozen scalari- 
form tracheids without any protoxylem-elements. As already 
mentioned, this central stele may be absent as a distinct 
vascular strand, appearing merely as a ridge on the inner 
border of the outer and cylindrical stele. It is perhaps worth 
while recalling in this connexion the structure of such a form 
