120 



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. 263. 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 



