DEGENERATION OF THE GAMETOPHYTE 405 



The beginnings of the degeneration of the gametophyte 

 became clearly evident in the pteridophytes when the rela- 

 tively small and simple prothallium took the place of the large 

 gametophytes, as illustrated in the Riccia and Marchantia 

 groups. Its further simplification was greatly accelerated by 

 heterospory, passing through four prominent stages: 



First. Dependence upon food stored in the microspore and 

 megaspore, together with gradual loss of chlorophyll, reduced 

 the gametophytes to small structures producing relatively few 

 sexual organs and gametes. Thus the gametophytes in the 

 pteridophytes became dependent upon food supplied by the spo- 

 rophytes by way of the spores, a relation exactly the reverse 

 of that in the bryophytes. 



Second. The gametophytes became differentiated as male and 

 female in sex, associated with the microspores and megaspores, 

 respectively. 



The spermatophytes, by means of the seed habit, brought 

 about the greatest changes in the gametophytes, as follows : 



Third. The female gametophyte degenerated to such an 

 extent by the retention of the megaspore (embryo sac) in the 

 megasporangium (nucellus) that the archegonium lost its form 

 and finally became represented in its essentials by the egg alone. 

 The vegetative tissue became reduced until only a few nuclei 

 of uncertain relationship (antipodal and polar nuclei) remain. 



Fourth. The male gametophyte degenerated in structure in 

 a similar manner until the antheridium disappeared, and the 

 numerous ciliated sperms of the pteridophytes were represented 

 by only two sperm nuclei, with associated protoplasm. Vegeta- 

 tive tissue was reduced until only a single nucleus remained in 

 the angiosperms to represent sterile cells of a male gametophyte. 



There arose, however, by means of the seed habit an activity 

 on the part of the male gametophyte which is one of the most 

 remarkable developments in plant evolution. The appearance of 

 the pollen tube, with its parasitic relations to the sporophyte, is 

 a very complex life relation. This was the chief cause of floral 



