OF CEYLON AND INDIA. 
421 
unlike in form to the leaf-like root-thalli of Dicræa Wallichii 
or some forms of D. stylosa. Another of these algæ, Deles- 
seria Leprieurii, is remarkably like Hydrobryum lichenoides 
or Farmeria metzgerioides, leaving out of consideration the 
comparatively insignificant secondary shoots of these forms. 
The leaf-like thallus of some of the Dicræas, again, is 
parallelled not only by the shoot thalli of Oenone and other 
South American forms, but also by the large leaves of many 
American forms, and all of these organs are in many respects 
closely similar to the thalli of many of the seaweeds. Still 
more curious forms of the Podostemaceous thallus occur as 
we have seen in Griffithella, but these are also parallelled in 
the Himanthalias and other algæ. The whole subject of 
these resemblances between plants far distant in relationship 
from one another is of course one of great difficulty, and it 
is impossible at present to do more than point out these very 
suggestive analogies of form which accompany analogy of 
the conditions of life, and which seem to indicate that an 
experimental and comparative morphological study of the 
forms of the Algæ and Podostemaceæ should be attended 
with interesting results. The parallellism also indicates that 
it is by no means unlikely that the extraordinary poly- 
morphism of the thallus, which occurs in certain genera and 
species of the Podostemaceæ, e.g.^ in Dicræa and Griffithella, 
may have its parallel among the algæ, and consequently that 
the latter group requires investigation from this point of 
view, to strengthen its taxonomic foundations. In both cases 
we are dealing with organs which are not hindered by a 
skeletal tissue from growing into almost any shape, and in 
which therefore a variety of form may be easily liable 
to occur with any slight change in the conditions of 
growth. 
Not merely do many of the Podostemaceæ present close 
similarities to the seaweeds, but the least modified group, the 
Tristicheæ, presents remarkable similarities in morphological 
features, and in the arrangement and anatomy of the leaves, 
to many mosses or liveifworts, especially to those of wet 
