286 WILLIS : MORPHOLOGY OF THE PODOSTEMACEÆ 
Speed of Current . — The next factor to be considered is the 
speed of the current of water in which the plants live. 
Leaving out of account the effects of scour, to be considered 
presently, there seems to be no speed of current in itself too 
great for the plants to be able to live in it. Podostemaceæ 
are frequently found in the very rapid currents flowing over 
the waterfalls and in the swiftest parts of rapids. On the 
other hand, it is not all the species that are able to live in the 
very swift water. The swifter the current, the greater the 
strain on the plants submerged in it, tending to drag them 
down stream. As might be expected, all the Podostemaceæ 
show very well adapted structures for resisting this strain. 
The very swift waters are inhabited, as a rule, only by the 
best adapted species, i.e., in general the most dwarfed and 
lichen-like ; the edges of waterfalls and the violent rapids in 
Ceylon and India are usually inhabited by the flat lichen- 
like Lawias and Hydrobryums, while the larger Dicræas, 
Griffithellas, and still more Tristicha, are found in places 
with less swift and violent water currents. As is already 
well known, the Podostemaceæ differ from other water 
plants ill the absence of the intercellular spaces which are so 
marked a feature in the latter, and this peculiarity is perhaps 
partly connected with the fact that they require to lie flat 
down upon the rocks to avoid the strain of the current as 
much as possible. A fragment of one of these plants at 
once sinks in water if thrown into it. 
Scour . — An important factor 'in the local distribution at 
any rate is the scour of the current, due to the quantity of 
suspended matter contained in the water, whether at all times, 
as in the Mahaweli-ganga, or only at seasons of high floods. 
If a very large scour goes on by the rolling of stones, pebbles, 
and gravel down stream, the Podostemaceæ are not able to 
survive this action, and places of much scour are found to 
be free of these plants, except in sheltered bays in the rocks. 
On t£e other hand, the plants may live successfully for many 
years in a particular locality, and may be exterminated or 
nearly so by a high flood causing an unusual scour. To such 
