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 



sp 



FIG. 39 (Heterosporous Ferns). A, sporophyte of Isoetes echinospom; B, 

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

 gium, ma ; the microsporangia 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 gametophyte, here reduced to a single archegonium, or; 

 the body of the macrospore, sp, remains undivided; F, spermatozoid 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- 



