OF CEYLON AND INDIA. 
409 
of the Dicræas and Griffithellas. The reason perhaps lies in 
the fact that the latter are usually free of the rock except at 
the base, and that they possess more capacity for growth in 
thickness of the cortical parts. This capacity, combined 
with an entire absence of any controlling skeletal tissue, 
such as is found in most flowering plants, or of fixed 
position, such as hampers the thalli of the closely attached 
Hydrobryums or Lawias, gives them a potentiality of varied 
form unexcelled above the somewhat similarly circum- 
stanced brown algæ. For details of the very simple way in 
which all the very varied forms are produced, reference 
must be made to the details given above under Griffithella 
and Dicræa. The effects are sufficiently striking ; it is 
difficult to realize that the objects figured in Plate XXVI. are 
really roots, or that most of them do not even differ varietally 
from one another. 
Another line seems to be that represented by Hydrobryum 
lichenoides and H. sessile, as well as by the Farmerias. 
This form of branched simple ribbon-like thallus differs 
from that of Podostemon subulatus and P. Barberi in its 
greater branching, very firm attachment to the rock, and 
greater flattening and dorsiventrality, with extreme reduc- 
tion of the secondary shoots. In Farmeriathe branching is 
again endogenous, as in the forms with which we started, 
though very likely this phenomenon is one acquired subse- 
quently in exogenously branched ancestors. 
In H. sessile we see the thallus branching in the same 
way, but wasting no space upon the rock by long gaps 
between the branches, and at times there is even a tendency 
for the sinuses between the lobes of the thallus to disappear 
by subsequent growth. Carry this feature a stage further 
and the deeply lobed liverwort-like thallus of H. Griffithii 
or H. olivaceum griseum is reached, and with a further 
development of the growth of the bases of the sinuses the 
almost circular-outlined thallus of H. olivaceum zeylanicum 
is formed. In these plants the term thallus is very 
eminently suitable to the organ in question, which hag 
