OF CEYLON AND INDIA. 
363 
growing points if again submerged. On the other hand, the 
reduction of the shoots to little tufts of leaves is attended 
with a corresponding loss of assimilatory capacity, but this 
is made up by the remarkable development of the thallus 
into a leaf-like structure, so that the mature plant is not 
unlike the large leafy Podostemaceæ of South America, 
though in the latter the leaves are of the ordinary kind. 
The thallus of Dicræa is still pretty evidently homologous 
with the root-thalli of the preceding genera, but regarded 
simply as a ‘‘root,” it is one of the most remarkable struc- 
tures that can be imagined under that category— endogenously 
developed from the main primary axis, it is true, and with a 
semblance of a root-cap, but exogenously branched, usually 
not attached to the rock, except at the base and perhaps else- 
where, but drifting freely in the water, and performing the 
chief functions of assimilation, besides bearing the florif erous 
shoots. 
Probably the drifting form of thallus should be regarded 
as derived from the creeping, and there is evidently as yet 
no very marked separation of the two types, except in D. 
elongata ; in the more algal types of Dicræa there is a good 
deal of polymorphism, and the thallus may assume almost 
any form, whether creeping or drifting. D. stylosa fucoides 
in particular is of interest in this way, and shows the first 
signs of the remarkable polymorphism which we shall see 
carried to extreme in Griffithella. 
The dorsiventrality of Dicræa is extreme, and shows itself 
in many features, such as the growth in thickness of the 
thallus, the structure of the vascular tissue, &c. It is notice- 
able, however, that it is not appreciably exhibited in the 
secondary shoots in their vegetative stage, though it is shown 
in the corresponding stages of Tristicha and Podostemon. 
Here we have perhaps a case in which the evolution has 
been through dorsiventrality back to radial symmetry. As 
soon as the secondary shoots become fiorif erous, they become 
dorsiventral once more, though not so markedly so as those 
(49) 
