OF CEYLON AND INDIA. 
•285 
during the gradually decreasing water depth and increasing 
light of the last months of the rains. 
That the plants have need for a considerable amount 
of light is shown by the fact that they are rarely to be found 
in shady places where the water is not exposed to the 
sunlight for at least some hours daily. They affect chiefly 
open sunny rapids. 
In this connection it may be mentioned here that nearly 
all these plants have a great amount of anthocyan in the 
surface cells, so that Avhen alive they have a red rather than 
a green colour. 
Light, as is well known, tends also to have a dwarfing 
effect on plants. Progressive dwarfing is one of the marked 
features of the evolution of the Podostemaceæ, but to what 
extent, if at all, light has operated in this must be a matter for 
future investigation. 
Temperature . — With regard to temperature, there is little 
to be said. The Podostemaceæ inhabit, with few exceptions, 
the tropical zone, where the temperature of the water is very 
uniform, and the few that live in more northern regions, 
e.^., those of Assam and Ohio, carry out their life-history in 
the summer months. The forms living in the low-country 
of Ceylon or South India have a very constant water 
temperature of about 80° F. (27° C.), and those of the hills in 
these countries a similarly constant though lower temper- 
ature, e.g.^ at Peradeniya of about 75° F. (24° C.), or at 
Paikara in the Nilgiris of about 58° F. (15° C.). In the Khasia 
Mountains of Assam the life-history is over by the time that 
the coldest weather begins, and the coldest water in which 
I found any species in that district in December, 1901, was 
about 57° F. (14° 0.). The temperature is so uniform in most, 
if not all, cases that it may probably be almost ignored 
as a morphological factor, but it appears to regulate the 
geographical distribution to a large extent. No Podoste- 
maceæ have yet been found in really cold water, and even 
in India and South America they are, so far as yet known, 
absent from streams fed by melting snow. 
