OF CEYLON AND INDIA. 
283 
and slight, and there is at any rate little prospect of the 
plants being actually exposed to the air, and that, too, on a 
substratum of bare rock rather than of water-retaining mud. 
In rapids and in mountain streams generally, however, the 
water-level is liable to frequent changes, whether of sudden 
rise or of fall sufficient to expose the plants to the sun and air 
on the bare rocks, or in very shallow water. It is by no means 
uncommon in August or September to find, at Hakinda, large 
quantities of Podostemaceæ exposed on the rocks by the fall 
of the water, and when exposed at this period the plants 
cannot form flowers, and simply die if not re-submerged 
within a short time. I am inclined to think that the gradual 
adaptation of the Indian Podostemaceæ to meet this danger 
has been one of the most marked features in their evolution. 
It seems to show especially in two ways, in the enormous 
capacity of rejuvenescence that the plants possess, and in the 
great dwarfing of many of the most highly evolved forms ; 
it probably shows also in the increase of number and 
diminution of size of the secondary shoots, and in the 
partial amphibiousness of the thalli. We shall return to 
this at the end of the paper. 
Ultimate Exposure . — Whatever may be the effect of the 
temporary changes of water-level, the plant must ultimately 
be completely exposed by the great fall of the water-level in 
the dry season, and adaptation of the life-history to this fact 
is an absolute necessity. This is clearly marked in the way 
in which the flowering takes place only at this time, while 
during the period of high water-level the plant is vegetative 
only, storing up large reserves of material to enable it to 
flower and ripen its seeds very quickly when the exposure 
does come, because, though able to live for a short time above 
water, the thalli are not usually capable of standing very 
long exposure. 
Aeratio7i.—A.noi\iQv important factor in the life-history is 
probably the aeration of the water. Instead of the almost 
stagnant water with a muddy bottom in which so many 
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