IN GYMNOSPERMS 41 
tends to become, and often actually is, a mere means of working up the 
material stored in the mature spore into gametes and an embryo, and 
does not possess any functional vegetative system. This is exemplified in 
Figs. 25, 26 of Se/aginella, which show the contents of the germinated 
microspore developed as little more than an antheridium. In Fig. 26 the 
wall of the spore is ruptured, and the contents are ready to be extruded 
as numerous spermatozoids. Fig. 24 a shows the megaspore with the 
female prothallus within it, bear- 
ing an archegonium. Fertilisation 
takes place as in Ferns through 
the medium of water. The ovum 
after fertilisation forms the embryo 
which remains for a time embedded 
in the prothallus: but later it bursts 
through, and establishes itself as 
the independent sporophyte. 
In many heterosporous plants 
the germination takes place after 
the spores are shed, just as is the 
case in homosporous plants. But 
in others germination of the mega- 
spore may be initiated or even 
carried through within the spor- 
angium. This is the case in 
Selaginella apus (Fig. 24), in 
which it is evident that, even 
when the sporangium has not yet 
opened, the prothallus may be 
well advanced in the megaspores. 
Fertilisation may be carried out 
within the sporangium after its tis ar due of feriiiston Xo. 2 embryersae 
filled by the prothallus; a, the venter; c, the neck of 
rupture, by means of spermato- an archegonium ;. 9, ovum: ny, its nucleus: ne, nucellus 
zoids derived from spores shed PNT cree a pollen-tubes ; 7, integument. 
from adjoining microsporangia, 
and the embryo may be developed while the megaspore is still within 
the sporangium. It is no great step from this condition to that seen in 
the Seed-Plants, in which the megaspore—or embryo-sac as it is called 
in Seed-Plants—remains embedded in the tissue of the megasporangium 
or ovule (Fig. 27). The physiological advantage gained by this step is 
an important one: there is no longer any need to hurriedly pass the 
nutritive supplies into the spore before its wall, thickened for protective 
purposes, stops the process of transfer; for in the Seed-Plants the wall 
of the megaspore, no longer needed for protection, remains thin, and the 
nutrition of the female prothallus can be continued until long after the 
embryo is initiated within it. 
Fic. 27. 
