390 THE SPERMATOPHYTES 



sporophyte to reach the embryo sac. These principles are asso- 

 ciated with the last stages in the long processes of the evolution 

 of the sporophyte and the degeneration of the gametophyte, 

 which is briefly outlined in the next chapter. 



Heterospory (Sec. 314, Chapter xxvn) has differentiated the 

 spores of the pteridophytes and established male and female 

 gametophytes, the first always developing from the microspores, 

 and the second from the rnegaspores. At the same time the 

 gametophytes became largely or wholly dependent upon food 

 material stored in the spores, and smaller and simpler in their 

 organization, until they degenerated into structures somewhat 

 similar to those now illustrated in the heterosporous pterido- 

 phytes (Marsilia, Selaginella, Isoetes, etc.). Some of them 

 finally lost all their chlorophyll, and adopted parasitic habits. 



Heterospory also resulted in the differentiation of the spore 

 leaves into microsporophylls and megasporophylls, and at last, 

 as in many seed plants, the sporophytes themselves became 

 differentiated, some producing only pollen (microspores) and 

 some only embryo sacs (rnegaspores) in the ovules. In this way 

 sexual characters of the gametophytes were gradually taken 

 up first by the sporophylls and later by the sporophytes them- 

 selves, and thus the asexual generation began to assume the 

 peculiarities of sex. The microsporophyll of the seed plant 

 (stamen) took on characteristics of a male organ, and the mega- 

 sporophyll (carpel) characteristics of a female one. The early 

 botanists regarded the pollen grain as a male element and the 

 stamen as a male organ, and it is true that these structures have 

 male characters ; but of course the actual male gametes are the 

 sperm nuclei with closely associated protoplasm in the pollen 

 tube whose contents represent a male gametophyte. And simi- 

 larly, although the carpel has female characters, the female 

 gamete is the egg within the embryo sac whose contents rep- 

 resent a female gametophyte. 1 



1 This subject is considered more at length in Chapter xxvn, Heter- 

 ospory, Sec. 339. 



