314 WILLIS : MORPHOLOGY OF THE PODOSTEMACEÆ 
formed between them, the tissue there being thin, like that 
at the edges of the original growing thallus. This thin tissue 
soon ceases to grow in thickness or length, and presently 
breaks, leaving an irregular gap between the two growing 
points. The seedling figured shows various phases of this 
process. It will be evident that the breaking away is 
not a very definite process, being sometimes very marked, 
sometimes hardly at all, as is indicated by the persistence of 
the large marginal leaves along the greater part of the gap. 
The cells of this thin tissue do not elongate so much as those 
in the main course of the thallus, though they develop the 
silica bodies. 
In PI. IX., fig. 11, a specimen is shown in which the 
branching is clearly visible at the tip, and it can be seen that 
there is one of the large marginal leaves exactly central 
between the branches, overlapped on each side by their 
marginal leaves. 
From this stage onwards the growth is evidently of the 
same type, and in August and September, by which time it 
is again possible to get material, the plants have reached a 
considerable size ; several such plants are shown in the 
photograph reproduced in PI. XII. The irregular but still 
definite manner in which the thallus has grown to this size 
is clearly evident. Several plants, it will be noticed, are 
growing side by side, and are already commencing to inter- 
fere with one another and to overlap, so that in the dry 
weather a few months later it will be very difficult to 
disentangle their mode of growth. It will also be noticed 
that as the plants get larger their apical portions get broader 
and more fan-like in shape. This is the expression of the 
fact that the branching or formation of new apices becomes 
more and more rapid in proportion to the rate at which by 
the elongation of the thallus these apices become separated 
each on its own piece of thallus. Along the edge of each of 
these fans there is a very large number of growing points, 
as shown in PI. XI., fig. 2, which represents a portion of the 
edge of the thallus of the common Ceylon form at the season 
