Cortex of Selaginella , Spr. 365 
S. caulescens, Spr. var. amoena, and S. emiliana. 
The very faintest trace of silica is to be found in the 
minute intercellular spaces between and on the innermost 
cortical cells. I could detect none, however, in S'. caulescens 
itself. 
Before any definite conclusions can be arrived at as to 
the part played by the siliceous deposit in the economy 
of the species that possess it, it will be necessary to cultivate 
specimens in an artificial soil from which silicates have been 
carefully excluded. In the absence of any experimental 
data on this aspect of the question, I can only at present 
suggest that the Si 0 2 is an excreted product, and that 
calcium and magnesium are absorbed, at least in great part, 
in the form of soluble silicates, the silica being eliminated 
in the insoluble form. What the agent in the decomposition 
is would also form an interesting question. I have en- 
deavoured to artificially produce decomposition of the silicate 
by hermetically sealing branches of S'. Martensii in a glass 
tube filled with water saturated with carbon dioxide. After 
a week’s exposure, however, I could detect no trace of silica 
in any situation save where it is to be found in the fresh 
condition. 
