330 WILLIS : MORPHOLOGY OF THE PODOSTBMAOBÆ 
greatest number of leaves that I have observe rl to develop 
upon any primary axis that I have seen, and the leaves are 
comparatively small. All the leaves appear, in one plane, 
i.e.y are distichous, with the cotyledons forming the lowest 
member of each rank. The leaves are hairy on the upper 
surfaces, and are usually, especially when young, of a deep 
reddish colour from the presence of anthocyan in the 
epidermal cells. 
The thallus appears almost at once (PI. XVI., fig. 2) 
growing out along the surface of the rock as a little papilla 
emerging from the basal portion of the hypocotyl, usually 
at right angles to the plane of the cotyledons. Its develop- 
ment is endogenous. Once formed, its growth is rapid ; 
it becomes attached to the rock by rhizoids and occasionally 
by haptera. Its growing point shows a peculiar structure, 
details of which must be reserved for future description. 
It has no deciduous root-cap whatever, but the extreme tip 
is formed by a collenchymatous layer beyond the initial 
meristem, similar to, but less marked than, that which we 
shall subsequently find in Hydrobryum and Farmeria. 
The “ epidermal ” tissue (if such it be) of the maturer 
parts of the thallus can be distinctly traced in surface view 
round the actual apex, being there continued by the collen- 
chyma. The extreme apical zone shows cells dividing in 
all directions ; then follows a zone of mainly transverse 
division, and then one of elongation. 
Very soon the thallus begins to branch. This it does 
very close indeed to the apex, and as might perhaps be 
expected from the construction of the latter, it branches 
exogenously. In PI. XVI., figs. 4, 5, the tip of a thallus is 
shown which has just branched, and other figures on the 
same plate also illustrate the branching. The branch forms 
close to the apex, so closely indeed at times that until it is 
observed that the endogenous shoot is also forming 
between it and the apex, it looks almost as if formed by an 
actual dichotomy ; in reality, however, the branching appears 
to be lateral. The shoot seems to be considerably developed 
