192 



MOSSES AND FERNS 



CHAP. 



cells of this central strand the broad midrib found in the mature 

 leaf is developed. In Amhlystegiwn all the cells of the midrib 

 are, alike and have thickened walls. The midrib projects on both 

 sides of the leaf, but rather more strongly upon the lower side. 

 In Funaria (Fig. loo), the structure of the midrib Is more 

 definite. Here two rows of cells take part in the formation of 

 the midrib. Each of these first divides as in Amblystegium by 

 a wall parallel to the surface of the leaf, so that in cross-section 

 the central part of the leaf shows a group of four cells, those 



Fig. gg.— 'Amblystegium riparium, var. fiuitans. A, Longitudinal section of the stem 

 passing through a young lateral branch {k) ; h, hair at the base of the subtending 

 leaf; B, horizontal sec+^^ion of a very young leaf, showing the apical cell (,x) ; C, 

 D, transverse sections of young leaves, showing the development of the midrib. 

 All the figures X525. 



on the outer side being larger than the others. In the former 

 the next wall is a periclinal one and divides the cell into an inner 

 and an outer one. From the two inner cells by further division 

 is formed the group of small conducting cells that traverse the 

 centre of the midrib, while the outside cells together with those 

 on the inner side of the midrib become much thickened and 

 serve for strengthening the leaf. As in Amblystegium the 

 lamina of the leaf remains single-layered, and its cells contain 

 numerous large chloroplasts which, as is well-known, continue 



