36 



MOSSES AND FERNS 



CHAP. 



the wall soon shows a separation into endospore and exospore. 

 The latter in R. hirta, which was especially studied, is very 

 thick, at first yellowish in colour, but deepening until when 

 ripe it is black. Sections parallel to the surface show in this 

 species what appear to be regular rounded pits, but vertical 

 sections of the spore-coat show that this appearance is due to 

 a peculiar folding of the exospore, which also shows a distinct 

 striation, the outer layer being much thicker and denser than 

 the inner ones. The nucleus of the ripe spore is remarkably 



Fig. 7. — Riccia, hhta (Aust.). A, Section of a spore mother cell undergoing its first division, x6oo ; 

 B, section of young spore tetrad, X 300 ; C, section of ripe spore, X 300 ; D, surface view of the exo- 

 spore of a similar stage, X 300. 



small, and it is evident that the dense contents of the ripe 

 spore is largely oil or some similar soluble matter, as in 

 microtome sections there is very little granular matter visible. 



At the same time that the first division wall forms in the 

 embryo, the outer cells of the venter begin to divide by 

 periclinal walls, so that the single layer of cells in the wall of 

 the unfertilised archegonium becomes changed into two, and 

 the basal portion becomes still thicker ; the neck takes no part 

 in this later growth. The cells of the venter develop a great 

 deal of chlorophyll, which is quite absent from the sporogonium 

 itself, and before the spores are ripe the inner layer of cells of 



