the Anatomy of the Genus Selaginella , Spr. 129 
adopted, although Dangeard’s method is perhaps the simpler 
of the two. It has the further advantage of being based on 
a somewhat more constant character, although neither the 
nature of the epidermis nor the composition of the mesophyll, 
singly, seems to me to afford a perfectly satisfactory basis. In 
the order of description of the species I have therefore com- 
bined these two characters. The great majority of the species 
which I have examined (thirty-two out of fifty-three) have the 
epidermal layers different in character, and show in section 
a reticulate mesophyll without any very definite palisade- 
layer. Three additional species with very delicate leaves 
have practically no mesophyll, but possess the epidermal 
characters of the same nature as those of the majority. The 
remaining species have the upper and under epidermis 
practically alike ; some, however, of these have, and others 
have not, a distinct palisade-layer. I have examined several 
hundred leaves, and find scarcely one of the characters I have 
mentioned at all constant. Even on the same epidermal 
surface, one finds elongated cells in one part and quite short 
polygonal cells in another. Again with regard to the meso- 
phyll, many species show a quite distinct tendency to form 
a palisade-layer (though not a compact one) where, on the 
whole, one would consider the mesophyll to consist of reti- 
culate parenchyma only. As for the stomata, these are most 
variable in their distribution, being situated sometimes on the 
ligular, sometimes on the aligular, and sometimes on both 
surfaces ; sometimes over the midrib only, sometimes on the 
wings, sometimes generally all over the surface. 
In the arrangement of the present paper I purpose first to 
give a brief account of the peculiarities of the leaves of each 
species, and thereafter to add one or two notes on special 
points, such as the structure and development of the stomata 
and the histology of the vascular bundle. I had occasion in 
a recent paper 1 to draw attention to the necessity of con- 
sidering the internal anatomy in the classification of the very 
1 On the diagnostic characters of the subgenera and species of Selaginella, Spr. : 
Trans. Biol. Soc. Liverpool, 1895. 
K 
