418 WILLIS : MORPHOLOGY OF THE PODOSTBMACBÆ 
chance of the plant as a whole, for some of them will have a 
chance of living, or even may never be exposed, and thus the 
life of the whole may be comparatively less injured than if 
there were only one or two shoots. 
Another advantage of the thallus, and especially when 
combined with a dwarfing of the secondary shoots, is that it 
enables the plant to live in swiftly running water without 
danger of being carried away. At the same time, the 
importance of this advantage may be easily exaggerated. 
Hydrobryum olivaceum, the species which inhabits the 
swiftest water of all, has a tall primary axis with long leaves 
at the period when the water is deepest and most violent, 
and this axis can hold fast to the rock if the rest of the 
thallus be taken away. Many of the South American forms 
have very large leaves and shoots, and yet live in very swift 
water. We must therefore beware of regarding the thallus 
too much as an adaptation to life in swiftly moving water. 
A third factor to be considered is the scour of the sus- 
pended matter in the water, which will be greater where 
the current is swifter, and which will probably be more 
injurious to the larger forms. 
It would seem, then, on the whole, most reasonable 
to regard the thallus with its secondary shoots, the chief 
morphological feature of most of the Podostemaceæ, as pro- 
bably adapted less to the rushing water as such than to the 
dangers inseparable from life in such water, chiefly the 
ever present risk of exposure by the shallowing of the water, 
but also perhaps the increased scour. The more highly 
modified the thallus, the swifter on the whole is the water 
in which it lives, as we shall see in detail in dealing with 
the geographical distribution of the forms. We may also, 
perhaps, regard as adapted to the same dangers the slight 
amphibiousness of most of the thalli (perhaps partly due 
to the silica that they so often contain), and their great 
capacity for rejuvenescence by forming new growing points 
behind any injured part, or even on older uninjured parts. 
