THE BRYOPHYTA : MUSCI, THE MOSSES 



435 



The Sporogonium 



The sporogonium consists of a foot, seta and capsule which are 

 developed in the early stages in the same way as those of Mnium, but the 

 foot is prolonged further down into the gametophore and appears to be in 

 more close contact with its tissues. The capsule is more highly specialized 

 than that of Mnium, and shows certain additional features (Fig. 423). The 

 seta and apophysis correspond in essentials to those of Mnium, though they 

 are considerably larger. The stomata on the apophysis have, however, no pore 



Operculum 

 Rim 



Amphithecium 

 Columella 



-/. 



^ 



Epiphragm 



Spore sac 



Apophysis 



Fig. 423. — Polytrichum coniniune. Median longitudinal 

 section of mature sporogonium. 



opening and appear to be functionless. The seta may reach a length of several 

 inches. The capsule, moreover, is square in section instead of being round. 



In the mature capsule the sporogenous tissue forms a tube around the 

 columella and is separated from it by air spaces which are traversed by 

 filaments of assimilatorv cells. A similar tissue is also developed between the 

 spore mass and the wall of the capsule. This additional assimilatory tissue, 

 or aerenchyma, as it is sometimes called, enables the capsule to obtain more 

 carbohydrate material from the air by photosynthesis, and it will be noticed 

 that since this tissue surrounds the spores they are in a very advantageous 

 position to receive the maximum amount of food material. 



The apex of the capsule also shows certain differences from that of 

 Mnium (Fig. 424). The calyptra, which is carried up by the elongating 

 seta, remains attached to the top of the capsule for a considerable time. It 

 develops a brown colour, and grows after its separation from the basal part 

 of the archegonium, forming a shaggy, hairy cap which covers the whole 

 capsule, whence the name " hair moss " is sometimes given to species of 

 Polytrichum. The operculum is conical, with a long beak or rostrum, and 



