FURTHER STUDIES ON GYMNOSPERMS. 



313 



only microsporangia. These leaves, while they resemble the ordinary leaves, 

 are smaller and correspond to the stamens. Upon 

 the under side, as shown in fig. 369, the microspo- 

 rangia are borne in groups of three or four, and these 

 contain the microspores, or pollen grains. The ar- 

 rangement of these microsporangia on the under side 

 of the cycas leaves bears a strong resemblance to the 

 arrangement of the sporangia on the under side of 

 the leaves of some ferns. 



631. The gingko tree is 

 another very interesting plant 

 belonging to this same group. 

 It is a relic of a genus which \ B / fll Fig. 370. 



Zamia inte- 

 r ifolia.show- 

 f n g thick 

 stem, fern-like 

 leaves, and 

 cone of male 

 flowers. 



flourished in the remote 

 past, and it is interesting 

 also because of the re- 

 semblance of the leaves 

 to some of the ferns like 

 adiantum, which sug- 

 gests that this form of 

 the leaf in gingko has 

 been inherited from some 

 fern-like ancestor. 



632. While the resem- 

 blance of the leaves of 

 someofthegymnosperms 

 to those of the ferns sug- 

 gests fern-like ancestors 

 for the members of this 

 group, there is stronger 

 evidence of such ances- 

 try in the fact that a pro- 

 thallium can well be de- 

 tig. 371. 



Two spermatozoids in end of pollen tube of cycas. (After termined m the ovules. 



The endosperm with its 



well-formed archegonia is to be considered a prothallium. 



633. Spermatozoids in some gymnosperms. But within the past two 



years it has been discovered in gingko, cycas, and zamia, all belonging to this 



