SEED PLANTS 159 
is begun. The microspores, however, although the 
germination begins within the sporangium, are finally 
discharged and complete their development outside 
the sporangium, precisely as in the Pteridophytes. 
The Spermatophytes do not differ in any essential 
structural points from the Pteridophytes. Like them 
they produce sporangia, usually upon special leaves 
(sporophylls), which are here known as carpels and 
stamens. Upon the former are borne macrosporangia 
(ovules), upon the latter the microsporangia (pollen- 
sacs). These sporangia agree closely in their structure 
and development with those of the higher Pteridophytes. 
In the microsporangium the development of the spores 
(pollen) corresponds in the minutest particulars with 
that of the microspores of the heterosporous Pterido- 
phytes, but in the macrosporangium, especially in the 
higher Spermatophytes, the Angiosperms, there is not 
always the division of the spore mother-cell into four 
daughter-spores. The macrospore in these forms is usu- 
ally known as the “ embryo-sac.” 
The development of the female gametophyte within 
the embryo-sac, especially in the lower types (Gym- 
nosperms), agrees very closely with that in Selaginella 
and Isoétes. After the germination is complete and the 
embryo has developed from the fertilized egg-cell of 
the archegonium, the wall of the macrosporangium 
hardens and forms a firm protective covering for the 
enclosed embryo, which generally is imbedded in the 
tissue of the gametophyte. The latter becomes filled 
with food-substances, such as oil, starch, and nitrogenous 
compounds, for the future growth of the embryo. The 
sporangium now falls away from the sporophyte, and is 
