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 
Fic. 39 (Heterosporous Ferns).— A, sporophyte of Isoétes echinospora; 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, ar; 
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- 
