150 



EVOLUTION OF PLANTS 



filled with accumulated food substances which serve to 

 supply the developing female gametophyte with food, 

 as the latter does not contain chlorophyll. The gam- 

 etophyte, as in Selaginella, is almost entirely included 

 within the large macrospore, and the formation of the 



Fig. 39 (Heterosporous Ferns). — A, sporophyte of Isoetes echinospora; B, 

 a single leaf showing the enlarged base bearing a single macrosporan- 

 gium, ma; the mierosporangia are much the same; C, a germinated 

 microspore with the contained gametophyte reduced to a single vegeta^ 

 tive cell, v, and an antheridium with four coiled spermatozoids ; D, 

 Marsilia vestita, a heterosporous form allied to the leptosporangiate 

 ferns ; sp, the " sporocarp " or modified leaf-segment within which are 

 borne the sporangia ; E, section of the upper part of the macrospore 

 and female gametopliyte, here reduced to a single archegonium, ar; 

 the body of the macrospore, .sp, remains undivided ; F, sperraatozoid of 

 Marsilia ; x, the remains of the central part of the sperm-cell. 



cells is preceded by a repeated division of the nuclei as 

 in the formation of the gametophyte or " endosperm " of 

 the flowering plants. Germination, however, does not 

 begin until the spores have been set free. The arche- 



