494 
Campbell . — The Development of 
narrower in front. It is not at all conspicuous, and is not 
always readily distinguishable from the adjacent segments. 
These are cut off from the lateral faces, apparently with con- 
siderable regularity, but their limits are soon lost. Comparing 
the horizontal section with the vertical section, it is evident 
that the form of the apical cell is that of a truncated wedge, 
from which four sets of segments are cut off, two lateral, one 
dorsal, and one ventral. It is in short exactly the same type 
as that of Riccia or Sphaero carpus. 
The Leaves. 
The leaves arise as outgrowths of the superficial cells of the 
lateral segments, and apparently each segment gives rise to 
a leaf. They stand vertical at first, but later become more or 
less shifted, so that they are finally inserted obliquely. In 
horizontal sections of the growing-point (Fig. 7), the growing 
leaf (/) appears as a simple row of cells. If a surface-view of 
the leaf is examined, however, it is then seen that the leaf is 
roundish and slightly pointed in outline. In some cases at 
least there was evidence of definite apical growth from a two- 
sided cell, much like what obtains in the similar leaf-like 
lobes of the prothallium of Equisetum maximum. In other 
instances it looked as if the first wall of the mother-cell of the 
leaf bisected it and was followed by others at right angles 
to it (Fig. 8). In such cases there is probably no definite 
apical growth. As the leaf enlarges, the marginal growth is 
unequal, and the older leaves are often decidedly irregular in 
outline and deeply lobed, so that their original arrangement 
is obscured. The development of leaf-like outgrowths from 
the dorsal side of the thallus, which are often partially con- 
fluent with the true leaves, disguise still further the two-ranked 
arrangement of the latter. These dorsal lamellae, as well as 
the leaves themselves, are for the most part but a single cell 
in thickness, but the base is usually thicker, merging somewhat 
gradually into the body of the thallus. 
In the original specimens sent from San Diego, the thallus 
