396 WILLTS : MORPHOLOGY OF THE PODOSTEMACEÆ 
and we find in H. olivacennij and perhaps in others, a com- 
paratively large primary axis at this period. By the time the 
risk of shallow water becomes great, the thallns is fully 
established and the plant can do without the large axis. We 
cannot regard the thallus as merely an adaptation for attach- 
ment to the rock, for the primary axis is able to hold on in 
the swiftest current if all the thallus be removed, except the 
part immediately at its base. 
In H. lichenoides we see the stage that would be reached 
by a simple dwarfing of the secondary shoots of Podostemon 
subulatus or better of P. Barberi. In H. sessile we get a 
further stage in the broadening of the thallus, so that it covers 
practically the entire surface of the rock included in its 
outer outline. In this species the sinuses at times tend to be 
obliterated by growth of their bases, and if this process be 
carried a stage further, we get the deeply lobed thallus of 
H. Griffithii and H. olivaceum griseum, and finally the 
shallowly lobed H. olivaceum zeylanicum, in which growth 
is no longer apical but marginal, and the root-cap is conti- 
nuous round the whole outer edge of the plant. This thallus 
shares with that of Dicræa the distinction of being probably 
the most remarkable organ yet described under the general 
morphological category of “ roots,” and were there not the 
series of stages connecting it to that of Tristicha and 
Podostemon, it might easily be looked upon as an organ of 
an entirely peculiar class. We shall return to this subject in 
the general summing up. 
FARMEBIA. 
[Willis, Rev. Pod. Ind., Ann. Perad., I., p. 246.] 
This genus, though evidently closely allied to Hydro- 
bryum, and resembling that in many points of structure, yet 
differs in certain very marked peculiarities, and is of 
particular interest. It is confined, so far as yet known, to 
Ceylon and the extreme south of India, in each of which 
there is a single species. The Ceylon species was described 
