400 WILLIS : MORPHOLOGY OF THE PODOSTEMACEÆ 
value to the thallus in that respect. The cells of this tissue, 
as they approach the edge, become longitudinally stretched 
(/.6., tangentially to the surface of the growing point), and 
now and then thin layers of them may be seen splitting off 
from the end of the thallus, just like an ordinary root-cap. 
The growth of the thallus is fairly rapid. As soon as it has 
grown a few millimetres out from the hypocotyl, it becomes 
attached by rhizoids to the substratum, and the endogenous 
leafy shoots begin to appear. PI. XXXYI., fig. 12, shows this 
stage : on the left side of the thallus, near the edge, a leafy 
shoot is just emerging, while a little nearer to tlie apex on 
the right-hand side is the swelling indicating the formation 
of a second younger one. The shoots appear with great 
regularity in this alternate and acropetal succession, when 
not interfered with by any obstacles, but when the substratum 
is very irregular, or there are many plants competing for 
space, the regularity may be lost. The next stage is exhibited 
in PL XXXVII., fig. 1. The primary axis shows the coty- 
ledons and four leaves, while the thallus, springing in a line 
with one of the cotyledons, has grown out to a considerable 
distance from the hypocotyl, and has developed several leafy 
shoots. At each node it is wider and also somewhat thicker 
than at the intervening part. This specimen also shows the 
branching of the thallus itself. The leafy shoots appear 
first, and are often, but not always, followed at the nodes by 
branching of the thallus. The shoots appear under two or 
three layers of cells at a very early stage. The appearance 
of the branches of the root-thallus is later, and it is often hard 
to determine whether they are really endogenous. Their 
development in the meristem remains to be Avorked out. 
They appear on the under side of the thallus, near to but not 
quite at the edge, just in front of the leafy shoot at the same 
node, as seen in PI. XXXVII., fig. 3. They are endogenous 
under a fe\v layers of cells. The branch thallus repeats the 
structure of the main thallus, and does not as a rule stop 
short in its growth like that of Podostemon subulatus, though 
it often becomes stopped by obstacles. 
