n6 
MORPHOLOGY 
and the liverworts. In these groups the thallus in its development 
passes through two or three stages, the first one being filamentous ; in 
the true mosses this earliest filamentous stage persists. This fact is 
associated with the development of the erect leafy branch (gametophore) 
characteristic of mosses. With green tissue displayed by erect branches 
the display of green tissue by the thallus body declines, and the 
thallus finally remains in an embryonic stage. Although the leafy 
branch is the con- 
spicuous part of 
mosses, it should 
not be thought of 
as the gametophyte, 
but as a branch of 
the gametophyte 
(fig. 251). In cer- 
tain mosses, known 
as reduced forms, 
this branch is not 
so prominent in its 
display of green 
tissue, only a few 
leaves ' appearing ; 
in fact it may bear 
only a single scale 
leaf in addition to 
the sex organs. 
The leafy branch 
FIGS. 251-254. True moss: 251, leafy branch arising . . , 
from proton ema and putting out rhizoids; a well-developed develops 
" resting bud " is shown ; 252, terminal rosette containing sex of an apical cell 
organs; 253, rosette containing archegonia; 254, a sp9ro- w j t ^ three Cutting 
phyte arising from a cluster of archegonia. 
faces. The segments 
are cut into outer and inner cells, the former, for the most part, 
developing the leaves ; the latter the axis. The leaves usually consist 
of a single plate of green cells, often thickened in the middle so as to 
resemble a midrib. 
Vegetative multiplication. The power of vegetative multiplication 
is remarkably developed. The leafy branch bears the sex organs 
above the moist substratum, so that the conditions are not favorable 
for swimming sperms. As a consequence, fertilization in many mosses 
254 
