33^ 



MUSCINEJE. 



the theca is ripe, it contracts and dries up at the bottom of the operculum, which 

 is formed essentially only of the epidermis ; or it remains attached to the columella and 

 forms a thickening at its summit, which projects over the opening of the theca; or 

 again it forms a kind of diaphragm, which closes the mouth of the theca after the 

 casting off of the operculum (Hymenostomum). The transition to the genera provided 

 with a true peristome is furnished by Tetraphis. In this genus the firm epidermis of the 

 upper conical part of the theca falls oif as the operculum, while the whole of the internal 

 tissue of the operculum, the two outer layers of which are thick-walled, splits across into 



Fig. 247. — h'linaria hy^'ronietrica 

 transverse section through the spore- 

 sac ; A, su the primary mother-cells ; 

 B, stn the spore-mother-cells not yet 

 isolated; a outer side, i inner side of 

 the spore-sac (XSSo). 



Fig. 248. — Development of the spores of Fjniaria hygrometrica observed in 

 very thin glycerine ; A mother-cells, at a still united, at b and c the separation has 

 commenced ; B isolated mother-cells clothed with cell-walls ; at f expelling the 

 protoplasmic contents ; C mother-cells with indication of the commencement of 

 the bipartition of the contents ; D the contents have divided into four lumps 

 of protoplasm, still surrounded by the primary cell-wall, but they themselves 

 are naked ; E the spores enveloped by cell-walls ; F ripe spores (X550). 



Fig. 249.— Various states of division of the mother-cells of the spores oi Fiotayia 

 hygrometrica, observed in water, the progress of development indicated by the 

 letters a—i. 



four valves. These are also termed by systematists a peristome, although their origin 

 and structure are widely different from that of the true peristome in other genera. 

 For, except in the Polytrichaceae, neither the teeth nor the cilia consist of cellular 

 tissue, but only of thickened and hardened parts of the walls of a layer of cells, which 

 is separated by some layers of thin-walled cells from the epidermis which forms the 

 operculum ; the latter layers, as well as the dehcate parts of the former, becoming 

 ruptured and disappearing, while the thickened parts of the wall remain after the 

 casting off of the operculum. This will be rendered clear by an example. Fig. 250 

 represents a part of the longitudinal section which bisects the theca of Funaria 



