THE EVOLUTION OF THE SPOROPHYTE. 15 



It is very certain that heterospory has developed quite inde- 

 pendently in all the -chief groups of Pteridophytes, and there is 

 evidence from the fossil remains of these plants that several 

 heterosporous types have become extinct. The multiple origin 

 of this peculiar phenomenon makes it exceedingly probable that 

 the formation of seeds, which is simply a further development 

 of the same phenomenon, has also originated more than once ; 

 and it is highly improbable that all of the seed-bearing or 

 flowering plants, to use a common term, have descended from 

 a common stock. It is much more in accordance with the 

 known facts to assume a multiple origin for the seed-bearing 

 plants or Spermatophytes. 



These highest of all plants do not possess any structures 

 which are peculiar to them. The flowers consist of one or 

 more sporophylls, or sporangial leaves, which are homologous 

 with those of the ferns. Upon these sporophylls stamens, 

 carpels are borne sporangia which are directly comparable 

 to those of the heterosporous Pteridophytes. The microspo- 

 rangia are here known as pollen sacs, and the macrosporangia 

 as ovules. The pollen sacs and spores agree in their most 

 minute particulars with the corresponding microsporangia and 

 spores of the heterosporous Pteridophytes. The spores in both 

 cases are shed, and the development of the very rudimentary 

 gametophyte takes place away from the sporophyte. The pol- 

 len spores always arise in tetrads, like the spores of all Arche- 

 goniates, and the structure of the ripe pollen spore is not 

 noticeably different from the spore of a liverwort or moss. 



With the macrospores the case is somewhat different. The 

 macrosporangium in the flowering plants differs in some respects 

 from that of the heterosporous Pteridophytes. Within it is 

 borne a single spore (embryo sac) which may be one of four 

 sister-cells, but is not necessarily so. The macrospore never 

 becomes free, but is retained permanently within the sporan- 

 gium, where it germinates, usually while the sporangium is still 

 attached to the sporophyte, but occasionally after it has become 

 separated. 



Within the macrospore, or embryo sac, is developed a rudi- 

 mentary female gametophyte, closely resembling that of some 



