342 WILLIS : MORPHOLOGY OP THE PODOSTEMACEÆ 
phyllotaxy that I have been able to discover. The primary 
axis is quite insignificant, and never reaches any size. 
The thallus appears almost at once. The base of the 
seedling axis forms a sort of tuber, as usual, which is fastened 
to the rock by root-hairs and by the formation at a very early 
stage of one or more exogenous hapfcera. The thallus 
appears to be endogenous, but in some cases that I have 
examined I was not able to satisfy myself that it was not 
exogenous. Great care has to be taken not to confuse thallus 
and hapteron in young seedlings ; the latter is always exo- 
genous and is usually larger-celled. PI. XVIII., fig. 1, shoAvs a 
young seedling with the thallus beginning to form. In fig. 3 
a rather later stage is shown on a larger scale to indicate the 
endogeny of the thallus, and the exogenous hapteron with 
its larger cells, which are now apparently more or less fully 
grown. A still later stage is shown in fig. 2. Here the 
primary axis has reached almost its maximum development, 
consisting of a few leaves on a very reduced stem. As soon 
as the thallus is well established the primary axis becomes 
quite an insignificant factor in the life of the plant, and in 
older plants cannot usually be found at all. The thallus 
groAVS along the rock, attached by root-hairs and by haptera 
for a distance of one or tAvo millimetres, then checks in its 
growth and forms a branch on the upper side, as seen in 
figs. 2, 4, This branch is the commencement of a floating 
or rather drifting thallus, Avhich extends upAvards into the 
Avater and does not, as a rule, become attached to the rock at 
any other point than its basal end. Usually the drifting 
thallus seems to start from the creeping one at a gentle 
foi-Avard slope or angle, as in fig. 2, but sometimes, as in 
fig. 4, it starts at right angles. These drifting thalli form 
the chief part of the mature plant, and being, like the 
creeping thallus, green, do a large part, if not the bulk, of 
the work of assimilation. The thallus groAvs rapidly by an 
apical growing point (PI. XVIII., fig. 5), already described 
by Warming, with a small terminal root-cap. A single 
Avascular strand runs up the thallus to near the tip, and 
