OP CEYLON AND INDIA. 
387 
that remains to the thallus of this species, though a series 
may be easily traced back to the root-thalli of such forms as 
Tristicha. As seen in transverse section, the thallus shows 
very similar features to those of most of the other genera 
with flat thalli that we have already examined. It has (PI. 
XXXIY., fig. 5) a well-marked epidermis and parenchy- 
matous cortical tissues, in the lower part of which the 
vascular bundles are to be found, showing the usual structure, 
but very small. The course of the vascular bundles in the 
thallus and their relation to the groAving margins is very 
complex, and I have not been able to make out satisfactorily 
any general principle. Each secondary shoot of course has 
a vascular bundle to it. 
The thallus does not grow in thickness like that of a 
Dicræa or a Grifhthella, though it becomes a little stouter 
behind the margin by the increase in size of the cortical 
cells. It is very sensitive to contact or to light or gra\dty, 
and folloAVS out every irregularity of the substratum. 
Before its diameter has reached more than a few milli- 
metres the thallus begins to branch ; this process seems to 
take place by a slowing of the growth in certain spots, and 
the thallus becomes lobed. When growing on smooth rock 
the thallus usually assumes a sort of trefoil shape in a short 
time, but very commonly there is no recognizable symmetry 
about it, as it grows to fit the more or less irregular sub- 
stratum, and usually comes into contact with other plants. 
Each lobe as formed continues to groAv in the same manner, 
and so does the tissue at the base of the fork between the 
lobes, so that, though these forks are at first quite close to the 
primary axis, later in the year there are none within several 
inches of it. The primary axis at first is at one side of the 
plant, but very often the thallus overlaps round it, or in many 
cases the tissue at the base of the stem grows out like the 
thallus, so that the stem stands more or less in the centre of 
a thallus. The lobes as they grow in size go on branching in 
the same way as at first, so that at last the appearance of a 
plant growing in an isolated position upon a large piece of 
‘ (52) 
