388 WILLIS : MORPHOLOGY OF THE PODOSTEMACEÆ 
smooth rock (PI. XXXV.) is like that of a large more or less 
circular piece of lichen, lobecl along the margin, the lobes 
being rarely more than an inch deep, and the division 
between them almost obliterated by the fact that the thallus in 
its growth does not diverge at the bases of the divisions, but 
if anything tends to converge, so that the lobes oftener than 
not overlap each other. The diameter of the thallus reaches 
15-18 cm. (6-7 in. 1 by the end of August (PI. XXXV.), and 
by January, when its growth finally ceases, often as much 
as 30-36 cm. (12-15 in.). The growing edge of the thallus is 
usually of a more or less deep red colour, while the mature 
part is perhaps most often of an olive green, but in dry 
weather, when the water falls so as to expose it to more 
intense light, it very often assumes, like all the other species, 
a reddish colour all over, due to the presence of anthocyan 
in the epidermal cells. As a rule, these plants have to grow 
on very irregular surfaces of rock, and so the thallus becomes 
equally irregular in shape. Often, too, a number of seed- 
lings commence close together upon a rock, and as they 
grow they collide with one another, and one grows over 
another ; when this is the case the lower thallus soon dies. 
Insect larvæ of various sorts feed largely upon the thalli, 
which contain enormous quantities of starch, and thus help 
to destroy their symmetry. If the water falls, as is not in- 
frequently the case in the drier end of the south-west mon- 
soon, in August and September, so as to expose the thallus, 
the exposed parts soon die, especially the growing margins, 
and when this is the case, the symmetry may be almost 
wholly lost; the growth is recommenced when the water rises 
again, in any part of the thallus that has not been killed, by 
a process of rejuvenescence described below, but the former 
symmetry is so far gone that little remains to show the 
principle of growth of the thallus. Hence, when the water 
finally exposes the plant in January, the principle of growth 
cannot be clearly made out at all, except by good fortune in 
finding plants that have not suffered any of the accidents 
we have described, and it is not surprising therefore that 
