316 WILLIS : MORPHOLOGY OP THE PODOSTEMACEÆ 
along the two closely approximated margins. This is faintly 
indicated even in the Ceylon plant figured in PL XII., where 
in the second plant from the top the “ hedges ” can be seen 
as white lines through the large fan-like lobe. 
The leaves require but little notice. They have been 
figured and described by Warming (42, IV., pp. 160, 163). 
They vary much in size in different forms, as well as on the 
upper surface and sides of the same growing point, but they 
are always moss-like leaves of very delicate texture, like the 
leaves on the ramuli of Tristicha ramosissima. They vary 
according to position on the thallus from linear to ovate or 
almost triangular, and in the lower part of each is a white 
mesial line showing very clearly in the living leaf ; the line 
has the corresponding linear or triangular shape to the leaf 
in which it occurs. Microscopic observation shows that the 
line represents a strand of narrow cells running up the 
middle of the leaf, and probably having some vascular 
function (PI. XI., fig. 3), while cross section of the leaf shows 
that in this central part the leaf is several cells thick, though 
the margin is only one cell thick (XI., 4). Silica bodies 
are common in the angles of the leaf -cells (PI. XI., fig. 3). 
The thallus itself has a simple structure as seen in cross 
section (PL XL, fig. 1). On the upper surface is a well- 
marked more or less columnar epidermis, and under this, 
forming the body of the thallus, is a large-celled parenchyma. 
Many of its cells contain large silica bodies, more especially 
those of the two uppermost layers. On the lower surface is 
a less conspicuous epidermis, bearing the rhizoids. The cells 
contain chlorophyll, and large quantities of starch. A little 
below the middle the vascular bundles take their course, 
which has already been described as seen in surface view (PL 
XI., fig. 2). The structure of the bundle is simple ; it consists 
of a few small cells, of which the upper are apparently of 
phlcem nature, the lower xy 1 em. I have not yet investigated 
the development of the bundle in detail. 
We have next to consider the development of the peculiar 
rosettes of leaves, which are found upon the older thalli, and 
