375 
Psilo turn fiaccidum , Wall. 
The leaves are borne on the ridges of the stem so that on the lower 
parts the leaf insertion is tristichous, and on the upper flattened parts 
distichous. 1 The sporophylls occur mostly on the upper branches, and are 
inserted just in the same position as the ordinary leaves. Among them, 
however, ordinary leaves m^.y be scattered, while on the other hand they 
are not confined to this region, and are sometimes found on the lower 
branches. The aerial shoot, like that of P. triquetrum , thus exhibits the 
‘ Selago ’ condition of Bower. 2 
The branching of the stem is apparently dichotomous, but Prantl 3 
showed that in P. triquetrum a leaf always occurred below the fork of the 
stem, and he supposed that one of the branches of the apparent dichotomy 
was really axillary. Solms-Laubach, on the other hand, thought that in 
P. fiaccidum the leaf insertion pointed to a true dichotomy, for the 
alternate distichous arrangement is continued at first on the two branches 
taken as a whole, as if no dichotomy had occurred, while the distichous 
arrangement on each branch is assumed higher up. 4 In my material of P. fiac¬ 
cidum there was constantly a leaf associated with the branching, although 
this was sometimes carried up, presumably by later intercalary growth, on 
to one of the branches. This point will be referred to subsequently, 
A single cylindrical stele traverses the stem and bifurcates with the 
latter. In many cases a vascular bundle is given off from the stem-stele to 
the leaves, though this is not always so. This would appear also to be 
much the same as in the case of P. triquetrum , 5 though, as many writers 
have stated that the leaves are without any vascular supply whatever in the 
latter species, leaf-traces are apparently of very much rarer occurrence in it. 
In P. fiaccidum^ as in P. triquetrum , the sporophyll is always served with 
a vascular supply. 
Internal Structure. 
i. Stem. At the lowest parts of the stems of which my material was 
composed, in what Miss Ford 6 describes for P. triquetrum as the inter¬ 
mediate region between rhizome and aerial stem, the branch is circular in 
transverse section and externally smooth and of a dark colour. The single 
stele is circular or somewhat elliptical in transverse section, and is sur¬ 
rounded by an endodermis which, although not very well differentiated, is 
yet clearly visible when suitably stained, its radial walls taking a lignine 
stain in some degree. The xylem consists of a band-shaped mass, generally 
with smaller protoxylem-groups at each end of the band, though sometimes 
these protoxylem-groups are not obvious. The whole of the xylem 
between the two protoxylems is composed of tracheae with scalariform 
1 Cf. Solms-Laubach (’ 84 ), p. 164. 2 Bower, F. O. (’ 08 ), pp. 165, 412. 
3 Prantl, K. (’ 76 ), p. 92. 4 Solms-Laubach (’ 84 ), p. 165. 
5 Pritzel, E. (’ 00 ), p. 616. 6 Miss Ford (’ 04 ), p. 598. 
