1 84 



MOSSES AND FERNS 



CHAP. 



(Fig. 87, D). By further divisions in all the cells of this central 

 strand the broad midrib found in the mature leaf is developed. 

 In Amblystegium 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. 88) 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 Amblystegiuni by a wall parallel 

 to the surface of the leaf, so that in cross-section the central 



Fig. 87. — Amblystegiuni rijiariinn, var. Jiuitans {Br. and Sch.). A, Longitudinal section of the stem 

 passing through a young lateral branch {k) ; h, hair at the base of the subtruding leaf; B, hori- 

 zontal section of a very young leaf, showing the apical cell {jc) ; C, D, transverse sections of young 

 leaves, showing the development of the midrib. All the figures X 525. 



part of the leaf shows a group of four cells, those 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 Here as in Amblystegium the 

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

 numerous large chloroplasts which, as is well known, continue 



