377 
Psilo turn flaccidum , Wall. 
At about the same level as that at which the stele begins to be 
hexarch, fibres similar to those found in P. triquetrum appear in the middle 
of the xylem. The simple pits in the walls of the fibres are easily seen. 
Occasionally parenchymatous cells are found among the tracheides (Fig. 4), 
and sometimes gaps may occur in the xylem-ring. The endodermis also 
becomes much clearer, and the thickenings on the radial walls are shown 
up very distinctly in sections stained with methyl violet and Bismarck 
brown. 
The cortex becomes more or less differentiated into an inner zone with 
fairly thin cell-walls, a middle zone of parenchymatous cells with somewhat 
thicker walls, and an outer assimilating layer. The epidermis here has 
numerous stomata. 
In this region of the stem the structure is therefore very similar to that 
of P. triquetrum . 
Sooner or later the number of protoxylems of the stele increase in 
preparation for the first bifurcation of the stem, where in all my specimens 
the fibres in the middle of the stem disappear. A leaf was constantly 
associated with this bifurcation, and in all cases examined received 
a vascular supply, such that practically one-half of the divided stem- 
stele was in the axil of the leaf-trace. Sometimes this leaf was carried 
up on to the fork of the stem above the bifurcation, but when this was the 
case its vascular supply originated in connexion with the bifurcation of the 
stem-stele. 
In the branches of a higher order central fibres were not observed, 
while in the flattened branches the number of protoxylem groups was con¬ 
stantly four, two of the six xylem arms of the triquetrous stems gradually 
dying out. Thus in P. flaccidum, except in the region of a bifurcation, and 
in the transitional region between the triangular and flattened stems, the 
number of xylem arms would appear to be double the number of ridges on 
the stem. 1 This is apparently a different state of affairs to that found in 
P. triquetrum , where any close connexion between the number of proto- 
xylem-groups and the number of ridges on the stem is not evident. The 
relationship in P. flaccidum is, moreover, often masked owing to the early 
preparation for the bifurcation of the stele. 
The xylem in the flattened branches occupies the centre of the cylin¬ 
drical stele, and is cruciform as seen in transverse section, the four proto- 
xylem-groups occurring at the end of each arm near the endodermis, so 
that the stele is here quite exarch (Fig. 6). The tetrarch arrangement was 
obvious in the smaller sporangiferous branches. 
The protoxylem consists of a few tracheidal elements at the ends of 
the xylem arms with spiral thickenings on their walls. The metaxylem- 
1 The transitional region between the hexarch and tetrarch stele is, however, generally of much 
greater extent than that between the triquetrous and flattened stem. 
