I2O 
MORPHOLOGY 
(fig. 263). This principal chlorophyll-bearing region of the sporophyte 
(apophysis) often is conspicuous, and sometimes becomes remarkably 
expanded. 
Seta. The seta is highly organized, with a central strand of elon- 
gated cells (not vascular); in fact it is the most highly differentiated 
axial structure below vascular plants 
(see figs. 1013-1016). 
The great groups. The principal 
groups of so large an assemblage of 
forms as the true mosses should be 
indicated, but it should be under- 
stood that they are extremely un- 
satisfactory, because they are very 
FIG. 963. True moss : longitudinal 
section through a mature capsule, show- 
ing operculum, peristome, columella, 
sporogenous tissue, complex wall of the 
capsule (conspicuous in which is the 
air-chamber region), and the apophysis 
(the region between the seta and the 
bottom of the air-chamber region). 
FIG. 264. True moss: structure of 
the operculum in longitudinal section; 
large cells in epidermal row belong to 
annulus; within are the thickening 
walls (shaded) of a layer of amphithe- 
cial cell, to form the peristome, which 
is anchored by special cells to the rim 
of the capsule below the annulus; three 
sporogenous cells (shaded) represent the 
top of the sporogenous layer, to the 
left of which is the large air-chamber 
developed in the amphithecium. 
After CHAMBERLAIN. 
artificial. The two main divisions are (i) Cleistocarpae (or cleisto- 
carps), characterized by the absence of an operculum, and hence with 
no peristome, the capsule opening by irregular rupture or decay ; and 
