VI. THE BRYALES 213 



extending from the lower to the upper surface of the stoma, 

 but with its ends ([uite free. The formation of the pore by 

 the sphtting- of tlic middle lamella of the division wall takes 

 place in the ordinary way. Later the walls of the epidermal 

 cells become very thick and show a distinct striation (Fig. 

 113). By the formation of the stomata the green assimilat- 

 ing tissue of the apophysis and central part of the capsule is 

 put into direct communication with the external atmosphere. 



The lower part of the seta grows downward and penetrates 

 the top of the stem of the gametophyte, from which, of course, 

 it derives a portion of its sustenance. The centre of the seta 

 is traversed by a well-marked central cylinder, whose inner 

 cells are small and thin-walled, and are mainly concerned in 

 conducting water; immediately outside of this is a circle of 

 thick-walled brown cells (leptome of Haberlandt), and the 

 rest of the seta is made up of nearly similar thick-walled cells 

 which grow smaller toward the periphery. 



At maturity, as the supply of water is cut off from below, 

 the capsule dries up, and all the delicate parenchyma compos- 

 ing the columella and inner part of the operculum, as well as 

 that between the spore-sac and the epidermis of the theca, com- 

 pletely collapses, leaving little except the spores themselves, and 

 the firm cell wxlls of the peristome, and the cells connecting 

 the latter with the wall of the capsule. By the breaking down 

 of the unthickened lateral and transverse walls of the peri- 

 stomial cells, the outer and inner thickened walls are separated 

 and form the two rows of membranaceous teeth that surround 

 the mouth of the urn (Fig. 114). By the drying up of the 

 thin-walled cells between the annulus and the margin of the 

 theca the operculum is loosened and is very easily separated. 

 The teeth of the peristome are extremely hygroscopic, and 

 probably assist in lifting ofT the operculum as well as removing 

 the spores from the urn. When wet they bend inward, extend- 

 ing into the cavity of the urn. As they dry they straighten 

 out and lift the spores out. The marked hygroscopic move- 

 ments of the seta also are no doubt connected with the dissem- 

 ination of the spores. 



The calyptra in the Bryales is very large and is carried 

 up on the top of the sporogonium in the form of a conspicuous 

 membranaceous cap. As in other forms it is the venter alone 

 that shows secondary growth. In Fnnaria it increases very 



