OF CEYLON AND INDIA. 
417 
origin the direct advantage of endogeny in many cases, and 
as another the physical necessity of it when the organ was 
developed at so late a period that the surface cells were no 
longer capable of full meristematic activity. 
Having now traced the morphology of the thallus through 
the very remarkable series of forms that it exhibits, we 
must go on to deal with it from an ecological point of 
view, and endeavour to trace the correlation between the 
morphology and the general conditions of life, or, in other 
words, to determine the amount and degree of adaptation 
displayed. 
One feature of great importance, common to all the 
thalli, is as we have seen the development of regular 
secondary shoots. In the simpler thalli, such as those of 
Tristicha and the American Podostemons, or even P. 
subulatus, this seems to be the only special function of the 
thallus as compared with an ordinary adventitious root, 
because the long creeping thread-like organ, with its 
comparatively slight hold of the rock, is not suited for 
purposes of anchorage of the long and complex secondary 
shoots. Anchorage of the latter, as soon as they have 
reached any considerable size, is provided for by the 
development of haptera. 
The function of the thallus seems rather to be to spread 
rapidly over the rocks, developing new shoots at short 
» 
distances, and holding them until they can form holdfasts 
of their own. Referring back to the general conditions of 
life, described in the introduction, we can easily see the 
advantages of this. If the water become shallow, as may 
easily occur at any time by a deficiency in the rainfall, the 
shoots are liable to be killed, or at any rate much injured, 
by exposure to the air, whereas the creeping thallus, which 
is also often more or less amphibious (or rather, capable of 
standing some exposure, for it never of itself emerges into 
the air), stands the best chance of surviving until a rise of 
water again occurs, and then forming new shoots. At the 
same time, the more shoots there are, the better for the 
