326 WILLIS : MORPHOLOGY OF THE PODOSTEMACEÆ 
water flowing over it. The actual distribution of this plant 
seems to point to the fact that this character is of great 
importance, and may be regarded perhaps as a direct adapta- 
tion to life in shallow water, or water liable to become 
shallow very rapidly. In Ceylon the species is distinctly 
infrequent, and it becomes more and more common 
proceeding further and further northwards through Western 
India, i.e,^ in going into regions in which the season of deep 
water becomes shorter and shorter. In the Khandala district 
the plant is enormously abundant, and is found in little 
brooks up to points quite near to their sources. In this district 
the rainy season is very short, but there is a trickle of water 
in the streams for some time, and the plants live in this. 
This reasoning is supported by the further fact that the 
other species of this district, Hydrobryum lichenoides, is 
also extremely dwarf. We shall return to the matter 
below. 
The great capacity of rejuvenescence possessed by this plant 
must also be of great advantage to it. It has an obvious 
advantage over Tristicha in the matter of the risks caused by 
fall of water-level at a non-lloriferous period ; exposure only 
kills a small part of it, instead of perhaps several large and 
complicated shoots, and owing to the fact that the thallus can 
bud out again from any part, growth is at once resumed, 
without the plant having suffered any serious loss. The 
thallus itself, too, is more or less amphibious, and 
can form new growing points even after considerable 
exposure. 
The endogenous secondary shoots are an interesting 
feature, but are of course not to be regarded as homologous 
with those of the root thalli of preceding genera. 
The mature characters of the plant appear very early 
indeed in the life-history, the dorsiventrality appearing 
almost immediately after germination. On the other 
hand, though the plant is so dorsiventral in its mor- 
phology and anatomy, the flower shows no sign of this 
character. 
