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 Isoetes. 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 



