OP CEYLON AND INDIA. 
433 
Hydrobryum (cf. Plates XXXI. , XXXITI.) would readily 
form fossils, which, if they had lost, as they probably 
would lose, their secondary shoots, might even be mistaken 
for liverworts. No fossil Podostemaceæ have as yet been 
described, but it is not unlikely that some may ultimately 
be found, perhaps in the mountains of North America or 
North-East Asia. 
Physiology. 
Cinder this head a few miscellaneous notes may be 
summed up, but the order still requires, and should repay, 
detailed physiological investigation on the spot. 
Nutrition probably takes place through the surface cells 
in the same way as in other water plants, the rhizoids 
appearing to be chiefly or entirely anchorage organs. An 
interesting feature in the nutrition is the great quantity of 
silica often present in the surface cells of the thalli, less 
often in the leaves or shoots. In such thalli as that of Lawia 
it is present in such quantity as to blunt a razor in cutting a 
single section. Whether this silica is taken up from the 
rock or from the water must at present remain an open 
question, also the further question whether it has any 
physiological function. It makes the thalli more rigid, and 
probably helps in preserving them from too rapid drying up 
when exposed to air. Those thalli which contain a large 
quantity of it are able to withstand considerable exposure 
and yet revive when again submerged. 
These plants expose large assimilating and absorbing 
surfaces, often further increased by the development of 
hairs on the surface of the leaves. The leaves show the 
general structure of shade leaves, and have usually chloro- 
phyll in the epidermis. Most of the forms have a great 
development of anthocyan in the surface cells, and 
consequently a red colour when alive. 
During the greater part of their life the Ceylon and Indian 
forms store up vast quantities of starch and other reserves in 
the thalli and bases of the secondary shoots, so that when the 
