ALTERNATION OF GENERATIONS 445 



gametophyte has reached the angiosperm condition in the final 

 eHmination of all prothallial cells. The female gametophyte, while 

 not so much reduced as the male, has nearly reached the angiosperm 

 level. 



The comparative display of the x and 2X generations may be 

 illustrated by a series of diagrams (fig. 397). The broader line rep- 

 resents the X generation, and the thinner line the 2X generation. 



In studying the alternation of generations it is interesting to note 

 that the gametophytes, in the early stages of phylogeny, were green 

 and independent, and that they later became parasitic and finally 

 lost their chlorophyll, while the sporophytes, originally green and 

 independent, became parasitic and lost their chlorophyll; then 

 later, in the bryophytes, pteridophytes and seed plants — after pass- 

 ing through a parasitic stage in their ontogeny — again became green 

 and independent. 



Alternation of generations, viewed as an alternation of x and 2X 

 phases in the life-history, is strictly antithetic. Since we beheve that 

 alternation of generations arose from the fusion of gametes, the re- 

 sulting zygote being the first cell from which the evolution of the 

 sporophyte began, we see no place for the old theory of homologous 

 alternation, especially since it is based largely upon the phenomena 

 of apogamy and apospory, which are merely abnormal digressions 

 from the normal life-history. 



In considering alternation of generations in the gymnosperms, it 

 is to be regretted that our knowledge of the fossil record is so in- 

 complete, especially knowledge of the gametophytic phase in the 

 life-history. It is to be hoped that some new discoveries of material 

 will show what the gametophytes were in the later heterosporous 

 pteridophytes and early gymnosperms which came from them. Until 

 facts are available, we can only imagine that the gametophyte 

 generation will become more and more extensive the farther back 

 the record goes, and that the sporophyte — in spite of Devonian trees 

 — will not be quite so far removed from a pteridophyte ancestry. 



