26 



MOSSES AND FERNS 



CHAP. 



In Riccia glauca, as well as other species, the uppermost cell of 

 each row often enlarges very much, and with its fellows in the 

 other rows constitutes the epidermis. According to Leitgeb's 

 researches this epidermal cell is formed by the first division in 

 the outer cell of the segment, and either undergoes no further 

 division, or by dividing once by a transverse wall forms a two- 

 layered epidermis ( R. BischoMi). On the ventral side the 

 odter cells of the segments project in much the same way, but 



Fig. 3. — Riccia glauca. Horizontal sections of the growing point. A, B, XS25; C, 

 Xabout 260. C shows the dichotomy of the growing point; x, x' , the two new 

 growing points; L, the lobe between them; ar, a young archegonium. 



they remain in close contact laterally with the neighboring cells, 

 so that instead of forming isolated rows of cells, transverse 

 plates or lamellae, occupying the median part of the lower sur- 

 face of the thallus, are formed. These remain but one cell 

 thick, and grow very rapidly, and bend up so as to completely 

 protect the growing point. With the rapid widening of the 

 thallus in the older parts these scales are torn asunder, and the 

 two halves being forced apart constitute the two rows of ventral 

 scales found in the older parts. Later these scales dry up and 



