422 WILLIS : MORPHOLOGY OP THE PODOSTEMACEÆ 
situations. Here again, comparative and experimental 
morphological study is required in both groups. 
To return from this digression, it would thus seem probable 
that the peculiar more or less expanded and assimilatory 
forms of the thalli (whether “shoot” or “root”) in these 
highly modified plants, unique as they are among the higher 
plants, and parallelled in the lower plants Avith more or less 
similar conditions of life, taken together with the reduction 
of the secondary shoots, are largely adaptations to the mode 
of life, more especially to the rising and falling water with 
the attendant risks of exposure. The thalli and secondary 
shoots of course shoAAq as Warming and Goebel have already 
pointed out, the common adaptations to life under water, 
such as flexibility of the drifting parts, absence of water- 
carrying or storing tissue except at the period of exposure 
for flowering, and so on. The most marked peculiarity, the 
absence of the large intercellular spaces, has been dealt with 
under Tristicha (p. 305), where it was mentioned that Goebel’s 
explanation is probably the correct one, viz., that as the water 
is fully aerated, there is no need for the air spaces which in 
many water plants are needed for the oxygenation of the 
lower parts. That the Podostemaceæ are very closely 
dependent on the motion or aeration of the water is shown 
by the fact that if placed in still Avater in a vessel, or if cut 
off in a pothole by the fall of the level of the river, they 
quickly die. 
Finally, we must briefly refer to the interesting stages 
shown in the life-history of the thalli, especially the changes 
that take place at the flowering season. In Ceylon they reach 
their full vegetative growth by about the end of October, and 
the secondary shoots begin to form their flowers in the last 
months of the year. By the time Avhen the level of the water 
falls at the end of December the flowers are quite ready to 
open. Further north of course the floral development is 
earlier, corresponding to the shorter rainy season. In 
Tristicha ramosissima the flowers are developed upon all or 
