NO. 584] 



HEREDITY AND ITS MEANING 



479 



apogamy in (Enothera f/if/as, and in Burner- the form with 

 the low number of chromosomes is apogamous while the 

 form with the high chromosome number requires fertili- 

 zation. On account of these exceptions, therefore, it 

 seems probable that the cause of apogamy is deeper than 

 a mere doubling of the chromosomes, even though douh- 

 ling may usually accompany such a change in reproduc- 

 tive habits. 



Variation in chromosome number in the same species 

 has been proposed as a cause of general variation in so- 

 matic characters, but the evidence is not clearly in favor 

 of such a theory. In the fern Xephrodium molle Yama- 

 nouchi found spermatid cells to be of two sorts, those with 

 sixty-six and those with sixty-four chromosomes. This 

 would mean that Xephroriium has two gametophyte forms 

 and two sporophyte forms, externally identical, so far as 

 our present knowledge goes, but differing in their chro- 

 mosome numbers. 



Further, sporophytes developing from the prothallia of 

 ferns without the intervention of a sexual process have 

 the N instead of the 2N chromosome number, yet apoga- 

 mously developed fern sporophytes, except as to chromo- 

 some number, are indistinguishable from normal sexually 

 produced individuals of the same species. 



Many writers have been tempted to postulate a causal 

 relation between the numerical variation of chromosomes 

 among the species of a genus and the genera of a family 

 and their specific and generic characters. The thirty or 

 more species of Composite investigated have shown a 

 remarkable variation in their chromosome numbers, the 

 2N numbers ranging between six and sixty, and, as is well 



genera, and the genus LUiuw, with its great variety of 

 characters distributed among forty-five species, is typical 

 of the other genera of the family, as far as present inves- 

 tigations go, in having the same chromosome number for 



