456 



MOSSES AND FERNS 



CHAP. 



The young spore has at first a very delicate cellulose 

 membrane, which thickens, and later has separated from the 

 outside the "middle layer" (Fig. 241, B, m), which in spores 

 placed in water lifts itself in folds from the underlying exospore. 

 The outer perinium seems to be unquestionably formed through 

 the agency of the nucleated protoplasm, in which the young 

 spores lie. It is at first a uniform membrane, closely applied 

 to the middle coat, but when placed in water it swells up and 



m.. 



Fig. 241. — A, Group of sporogenous cells, just before the final division into the spores, embedded in 

 the nucleated protoplasm formed from the disintegrated tapetum, and sterile archesporial cells, 

 X500 ; B, optical section of young spore, showing the three membranes ; in, the middle lamella, 

 X500; C, an older spore, showing the splitting of the outermost coat to form the elaters, X500 ; 

 D, surface view of the dorsal cells of the wall of a ripe sporangium, X 150 ; E, section of the wall, 

 showing the remains of the inner layers of cells {t), X 250. 



separates completely from the exospore, or remains attached to 

 it at one point only, which marks the point of attachment of 

 the elaters in the ripe spores. The elaters arise from the 

 epispore by its splitting spirally into four bands (Fig. 241, C), 

 due apparently to thickening along these bands, leaving thin 

 places between, which are finally absorbed. The outside of 

 the elaters becomes cuticularised. The ripe spores contain 

 numerous chloroplasts, which only are evident in the latest 

 stages of development. In E. arvense the formation of the 



