282 WILLIS : MORPHOLOGY OF THE PODOSTBMACBÆ 
(yoL II., p. 314, note), says under date September, 1850 : — 
“ Podostemon grew on the stones at the bottom : it is a 
remarkable water plant, resembling a liverwort in its mode of 
growth. Several species occur at different elevations in the 
Khasia, and appear only in autumn, when they often carpet 
the bottoms of the streams with green. In spring and summer 
no traces of them are seen ; and it is difficult to conceive what 
becomes of the seeds in the interval, and how these which are 
well knov/n, and have no apparent provision for the purpose, 
attach themselves to the smooth rocks at the bottom of the 
torrents. All the kinds flower and ripen their seeds under 
water, the stamens and pistil being protected by the closed 
flower from the wet. This genus does not inhabit the 
Sikkim rivers, probably owing to the great change of 
temperature to which these are subject.” 
The specimens which were collected by Sir J. D. Hooker 
at this time were in flower or fruit. Whether the statement 
that the flowers are fertilized under water is always correct 
or not should be settled upon the spot. My own visit was too 
late in the year for this purpose, but I saw evidence leading 
me to suppose that in Dicræa Wallichii there might have been 
cleistogamic fertilization. In the Hydrobryums I did not see 
any evidence of cleistogamy. Cleistogamic flowers occur in 
Podostemon Barberi, so that there is no reason to doubt that 
they may occur in others of the order, and indeed one is only 
surprised that this is not a more common phenomenon. 
We have thus seen that the depth of the water at the various 
times of the year, itself regulated by the distribution of the 
rainfall, is the immediate determinant of the phases of the 
life-history, and we must now go on to consider the other 
chief general factors in the life of these plants. 
Temporary Variations in Depth of Water . — Not merely 
does the grand period, as we may call it, of the water depth 
determine the great outlines of the life-history, but the more 
temporary variations in depth have a most important effect 
on the ecology of these plants. In the slowly moving streams 
affected by most water plants the changes of level are slow 
