493 
Geothallus tuber osits , Campbell . 
dally when the plants are growing under glass, and give 
the plant an exceedingly characteristic appearance. In 
addition to the marginal lamellae or leaves, there are 
frequently formed similar outgrowths upon the dorsal surface, 
which, however, do not appear to bear any direct relation to 
the leaves, and are extremely variable in number, size, and 
position. 
Apical Growth of the Thallus. 
A vertical section through the apex of the thallus, at any 
stage, shows the free surface of the growing-point to be nearly 
vertical, and near the middle is the apical cell (Fig. 6, x ), 
which is small when compared with the typical anacrogynous 
Jungermanniaceae, and recalls more nearly, perhaps, that of 
the Marchantiaceae. It is deeper, however, than is usual in 
those forms, or in Sphaerocarpus , which also has the apical 
cell of the same type and is not so near the ventral side of the 
thallus. Alternate dorsal and ventral segments are cut off 
from this cell in the usual way ; but on account of the rapid 
secondary divisions in them, the limits of the segments are 
much sooner lost than in Sphaerocarpus. Some of the super- 
ficial cells of the vertical segments form jointed glandular 
hairs, like those commonly found in the anacrogynous Junger- 
manniaceae. They are much larger than the corresponding 
ones in Sphaerocarpus , and are not usually formed so near the 
apex. They lie on either side of the median line, so that an 
exact median section may fail to show them. Each hair 
(Fig. 9) consists usually of a small basal cell, and a row 
of several others, ranging from two or three to five or six 
in exceptional cases. The terminal cell is enlarged, somewhat 
pear-shaped in outline, and . filled with densely granular con- 
tents. The secretion of this cell is probably mucilaginous, 
and the whole cell in the older hairs stains very strongly with 
Bismarck-brown. No doubt the mucilaginous secretion serves 
as a protection for the growing-point against evaporation. 
In sections of the growing-point made horizontally, the 
apical cell appears four-sided in outline, often somewhat 
