the A natomy of the Genus Selaginella , Spr. 1 5 1 
same leaf-surface one finds polygonal cells near the stomata 
over the midrib, whilst on the margins and base the cells are 
of the elongated type. 
4. The Me sophy It. 
The mesophyll varies also very greatly in amount, and also 
in the form of its component elements. In general the main 
mass of the leaf consists of a reticulum of longer or shorter 
cells (compare Figs. 18 and 29) with comparatively large 
intercellular spaces. In certain species a pseudo-palisade-layer 
is formed, either of the terminal superficial cells of the reticu- 
lum (Fig. 23), or of special smaller cells connected with the 
reticulum (Fig. 21). In other cases, again, a quite definite 
palisade-layer is formed, as in 5 . concinna (Fig. 25) and 
vS. Lyallii (Fig. 17). Towards the edges of most leaves, and 
in some cases (e.g. 5 . molliceps , &c.) throughout the entire 
leaf (save immediately round the vascular bundle), the 
mesophyll is entirely wanting, and the two epidermal layers 
are in contact. In such cases the guard-cells of the stomata 
are very shallow, and since the aligular epidermal cells are 
deep, a well-marked respiratory and transpiratory cavity is 
left, even in a leaf only two cells thick. 
5*‘ The Vascular Bundle. 
The vascular bundle is almost invariably simple, and 
extends on an average through five-sixths of the length of the 
leaf. In some cases it reaches to the base of the cusp, but 
never (so far as I have seen) enters it. In one or two cases, 
e.g. 5 . molliceps , I have found a bifurcate midrib, but this so 
rarely that I am forced to believe that the bifurcate condition 
is to be explained as a mere sport. 
In structure the bundle is very simple. In S. Kraussiana , 
for instance, the xylem of the midrib near the base and as far 
as the middle of the leaf consists of four to five delicate 
tracheides, of which as a rule one is annular, the rest spiral, 
the spiral being the first to appear. In the upper third 
of the leaf the protoxylem-elements become accompanied 
by several short reticulate tracheides which flank the 
