379 
Psiloturn flacciduni , Wall. 
central part of the walls of these cells appears to be much more lignified 
than the part bordering on the lumen, which in some cases would appear 
not to be lignified at all. The cells are elongated longitudinally, and have 
pointed ends, the latter being much more lignified than the rest of the 
wall. Simple pits are very numerous on the walls of these cells, and are 
easily observed. 
On the narrow diameter of the stem the thickened layer abuts directly 
on the chlorophyllous assimilating layer (Fig. 8). This extends completely 
round the stem as a layer of from two to four cells wide just inside the 
epidermis, and filling up all the space between the epidermis at the ends 
of the longer diameter of the latter. In longitudinal section these cells 
show the peculiar shape already noted for those of this tissue in P. tri¬ 
quetrum . 1 The walls are thin and wavy in contour, touching the adjacent 
cells in three or four places only, so that intercellular spaces between the 
cells are large and numerous. 
Between the chlorophyll-containing tissue in the wings and the fibrous 
layer round the bundle are parenchymatous cells, which pass gradually 
into the latter. 
The epidermis is a single layer of regular cells with a very thick 
outer wall; as in P. triquetrum 2 the innermost layer of the outer wall is not 
cuticularized, while the outer layers become more cuticularized the further 
they are from the inner one. Under high magnifications the layering of the 
outer wall is very distinct. The epidermal cells are somewhat elongated 
longitudinally. 
Stomata are numerous, as would be expected in an assimilating stem ; 
they are sunk somewhat below the outer level of the epidermis. They 
are somewhat peculiar in shape, there being only a single ridge on each 
guard-cell, a state of affairs which is somewhat rare. 3 In P. triquetrum , 
according to De Bary, both ridge of entrance and ridge of exit are absent. 
2. Leaf-trace. When a leaf-trace is given off from the stele the 
xylem arm on the side of the leaf becomes elongated, and ultimately two 
or three tracheides, or sometimes more, are detached, which pass up to the 
leaf, making an angle of about 30° with the main stele (Fig. 9). These 
tracheae are small, and have spiral or scalariform thickenings on their 
walls. As a rule the number of elements is too small to enable one to 
determine the position of the protoxylem. Surrounding the tracheae are 
some elongated thin-walled cells continuous with those of the phloem of 
the stem-stele. In some cases a distinct endodermis was observed. 
3. The Leaf. The leaves are small lanceolate structures about one 
millimetre long, and similar in shape to those of P. triquetrum (Fig. 10). 
There is an epidermis with a very thick cuticle, this layer being continuous 
1 Ford, S. O. (’ 04 ), p. 592. 2 De Bary, A. (’ 84 ), p. 77. 
3 De Bary, A. (’ 84 ), p. 35. 
