DEVELOPMENT OF PLANTS 291 



while the upper portion elongates by means of an apical cell 

 (Fig. 209, 10). Later, the growth becomes basal as in Antho- 

 ceros. For a time, the archegonium keeps pace with the elonga- 

 tion of the young sporophyte (Fig. 209, ii), but finally it is 

 ruptured and lifted up as a cap, called the calyptra, on the apex 

 of the young sporophyte. This is one of the most essential 

 differences between the mosses and hepatics. In the latter group, 

 with the exception of the Anthocerotales, the spores were ma- 

 tured in the archegonium. In the Bryales, the spores are usually 

 not formed until the seta has elongated considerably. As the 

 sporophyte elongates, the upper part enlarges and finally forms 

 a complex capsule which remains covered for a varying length 

 of time by the calyptra. The hairy calyptra of Polytrichum 

 (this name means many hairs) is due to the development of a 

 felt of hairs upon the archegonium which forms a protective cover- 

 ing to the young sporophyte against the loss of water. 



The capsule and calyptra assume various forms and positions. 

 In some cases the capsule is quite erect and completely covered 

 by the calyptra (Fig. 210, i), and again it may be more or 

 less inclined or even pendulous and the calyptra laterally placed 

 like a cap (Figs. 205, A; 211, 5). Removing the calyptra you 

 notice that a lid or operculum is situated on top of the capsule 

 while at the bottom is a somewhat enlarged region, the apophysis, 

 provided with stomata that communicate with the interior re- 

 gions of the capsule (Figs. 210, 2; 211, 5). When the opercu- 

 lum is removed a circle of minute teeth-like structures, the peri- 

 stome, appears that more or less completely closes the mouth of 

 the capsule (Fig. 205, D). In some mosses a delicate membrane, 

 the epiphragm, lies just below the peristome, which assists in 

 closing the mouth of the capsule (Fig. 210, 2B, 4A). We are 

 now interested to know the meaning of these structures and to 

 learn how they have come about. By examining a longitudinal 

 section of the nearly mature capsule that has been freed of its 

 calyptra you will see that the spore-producing tissue is relatively 

 small and appears as a hollow cylinder instead of a dome-shaped 

 zone of cells (Figs. 210, 3; 211, 6). This sporogenous tissue 

 is usually surrounded on the outside, and less commonly on the 



