OP CEYLON AND INDIA. 
287 
a cause, I think, roust be attributed the fact that the river 
below Toong in the Sikkim Himalaya, which in 1899 
contained large quantities of Hydrobryum Griffithii, now 
appears to contain none ; the great cyclonic storm, on the 
occasion of the Darjiling landslips, must have caused an 
immense scour. So also, though I searched the stream in 
which Grifiath originally found the species just mentioned, 
I was quite unable to find a single plant ; it seems to have 
been exterminated there since 1835 by natural causes. 
Substratum . — As might be expected, the common sub- 
stratum upon which these plants are found is smooth water- 
worn rock, but occasionally, when a log of wood or other 
object has become firmly wedged among the rocks, plants 
may also be found upon it. They are never found on 
unstable substrata, with the exception of one or two species 
which grow in slower water, and are found attached to 
pebbles, which they usually fasten together by their creeping 
thalli. This is often the case with the Indian Tristicha 
ramosissima and the North American Podostemon Cerato- 
phyllum. The actual composition of the rock seems to be a 
matter of practical indifference, and it is very doubtful if 
the plants absorb much or any food from the rock upon which 
they grow, unless perhaps silica, with which they are often 
very largely provided. 
Deposit . — A considerable amount of silt is often deposited 
on these plants, especially at times when the speed of the 
current slackens and the water-level falls. It must affect 
their assimilation, and at times is heavy enough to injure or 
kill them. 
Biological Factors . — Competition with other plants is a 
factor of very slight importance indeed with all the more 
modified Podostemaceæ ; they live in water so rapid that no 
other flowering plant, and only occasionally any Cryptogam 
other than a minute alga or two, is found there. The less 
modified forms, such as Tristicha, oftep share their habitat to 
some extent with aquatic mosses, and on rare occasions with a 
