450 WILLIS : MORPHOLOGY OF THE PODOSTEMACBÆ 
The Geographical Distribution of the Indian 
Podostemaceæ . 
Owing to their being confined to inonntain streams, and to 
their somewhat imperfect mechanism for seed-dispersal, the 
Podostemaceæ are very local in their distribution, and as has 
already been several times mentioned, each river or group of 
rivers or even each section of each large river often possesses 
its own local forms or species of each genus represented in it. 
With a complete knowledge of the forms in all the mountain 
streams of the tropics, it should be possible therefore to 
arrive at conclusions of great importance as to geographical 
distribution within these regions, but at present only the 
beginnings of such knowledge are at our command. 
The only method of distribution which appears to be of 
any avail to carry these plants from one river to another is 
the adhesion of the sticky seeds to the wet feet of wading 
birds ; these may often be seen in the dry season walking 
over the thalli, and the seeds must cling to their feet. It is 
therefore unlikely that any carriage of the seeds over more 
than a few hundred miles ever takes place, and in general 
the distances travelled will be much less. As in India and 
Ceylon the flowering takes place in the north-east monsoon, 
and as it is easier, in all probability, for a plant to suit itself 
to a longer than to a shorter vegetative period, it would seem 
not unlikely that the general migration may be southwards 
rather than northwards ; but there are numerous other 
factors to be taken into consideration, and it is not safe to 
say more than that perhaps this is the tendency at the present 
time. 
The order is practically confined to the tropical regions 
of the globe, and only’ passes into the warm temperate zones 
in a few districts. The local distribution of the forms 
is of considerable interest, and helps to throw light upon 
their evolution and adaptation. So far as India and Ceylon 
are concerned, we find that in general the larger forms 
