234 THE SIMPLEST LAND PLANTS 



tissue, but there is little differentiation in the structure 

 of the cells. In the surface cells starch grains may be 

 detected enclosed in the chloroplasts if the plant has 

 been exposed to fairly bright light ; in the central 

 cells there are large starch grains which have been 

 formed by chloroplasts, out of which they have burst, 

 and which may sometimes be still detected as green 

 smears on the surfaces of the grains (cf. Pellionia, 

 p. 126). The rhizoids are seen to be tubular outgrowths 

 of the cells on the lower surface of the midrib. The 

 midrib passes gradually into the wings, which at their 

 outer edges are only one cell thick. Pellia, like Fucus, 

 grows by division of apical cells at the tips of the 

 branches of the thallus. The cells cut off from these 

 divide further to form the whole of the tissue of the 

 thallus. 



Reproduction and Life History. — Pellia, like Fucus, 

 and also like all the higher plants, is reproduced by 

 means of sexually differentiated gametes ; and as in 

 Fucus these are formed in sexual organs arising from 

 surface cells of the thallus. There is an important 

 difference, however : in Pellia (as in all the Liverworts 

 and Mosses and in the higher group called Pteridophyta) 

 the sexual organ, both male and female, is covered in 

 the mature state by a wall consisting of a layer of 

 cells (Fig. 37, A, C, w), instead of by a cell wall only 

 as in Fucus. This we may perhaps relate to the 

 greater protection from evaporation required by sub- 

 aerial life. The antheridia (Fig. 37, ^) are spherical 

 structures formed on the upper surface of the thallus, 

 each in a little cavity due to the arrest of cell divi- 

 sion in the thallus tissue just below the spot where 

 the antheridium is formed and the growing out of 

 the thallus cells to roof in the antheridium on each 



