348 ALTERNATION OF GENERATIONS 



The chromosomes are generally believed to be the actual 

 bearers of the qualities (represented perhaps by rudiments) 

 which are inherited, that is, passed on from one generation to the 

 next. The chief reasons for this view are their importance as 

 the essential structures of the nucleus, their regular behavior 

 throughout the cell divisions, and the evidence that they never 

 lose their identity completely, even in the resting nucleus, but 

 remain perhaps as the only permanent organs in the cell. 



335. The life-history formula, showing the chromosome 

 count. The life-history formula which has been employed for 

 the bryophytes and pteridophytes becomes much more interest- 

 ing when considered in reference to the chromosome count. 

 The formula has been : 



Gametophyte <^ ^> Sporophyte asexual spore 



e 99 



Gametophyte^ etc. 



Representing the gametophyte number of chromosomes by x 

 and the sporophyte number by 2x, these may accompany the 

 formula as follows : 



sperm 



Gametophyte <^ * chro - ^> Sporophyte asexual spore 

 x chromosomes ^^ Zffff ^ 2 x chromosomes x chromosomes 

 xchro. 



Gametophyte, etc. 



x chromosomes 



Examining this formula, it is clear that there are two periods 

 when the number of chromosomes changes abruptly: (1) at fer- 

 tilization, when the number is doubled, and (2) at spore forma- 

 tion, when the number is reduced. The fertilized egg develops 

 into the sporophyte because its protoplasm has different qualities 

 from that of the gametophyte. The asexual spore develops into 

 the gametophyte because its qualities have become again the same 

 as those of the former gametophyte generation. Spore formation, 

 then, in bryophytes and pteridophytes is a return of the plant 

 in its life history to the conditions of ancestral gametophytes. 





