OF CEYLON AND INDIA. 
419 
The thallus, then, appears to be well adapted to such a 
mode of life, but we are now met as usual with the question 
whether the adaptation is a direct one, or only a perfecting 
of characteristics already existing in the ancestral forms, and 
which perhaps enabled them in the first place to adopt this 
peculiar mode of life. Here, as usual, we are checked in 
our inquiry by our ignorance of the phylogeny, and still 
more in this particular case by the absence of other families 
of flowering plants living in similar conditions, with which 
comparisons can be made. Neither the development of 
adventitious shoots on the root nor the regeneration of the 
growing points is at all uncommon in other families, and 
hence it seems very probable that both of these characters 
were well marked in the ancestral forms of the Podostema- 
ceæ. But they are evidently of great value to these plants, 
and have become developed to a great degree of perfection, 
and may thus perhaps best be regarded as partial rather 
than absolute adaptations to the mode of life. 
While the thallus, regarded broadly, is thus apparently 
not a direct adaptation, it is by no means improbable that 
there may be in it many partial direct adaptations, and we 
must now trace it a step further into the more peculiar forms, 
such as the Dicræas and Hydrobryums. In these we begin 
to find forms unique among the higher plants, and may 
therefore look for more evidence of direct adaptation. 
We have seen that the peculiar expansion of the thallus 
so marked in these highly modified forms is accompanied 
by a dwarfing of the secondary shoots, and perhaps on the 
whole by a slight increase in their numbers. It is easy to see 
that the dwarfing is in some ways advantageous to the plant, 
as it decreases the risk of exposure to the air when the water 
becomes shallow, but, on the other hand, it also decreases the 
assimilatory area and capacity of the plant, and the number 
of flowers borne on each shoot. Increase in the number of 
the shoots compensates more or less for these losses in some 
cases, and also such features as the development of larger 
leaves in Podostemon subu latus or Hvdrobryum olivaceum, 
(56) 
