518 



THE AMEBIC AN NATURALIST 



[Vol. L 



is more probable, however, than that the chromosomes 

 are qualitatively different, and that the gig as characters 

 depend upon the duplication of some, not all, of the chro- 

 mosomes. An irregular reduction division might well 

 result in the retention in duplicate of those particular 

 chromosomes upon which the gigas characters depend. 

 The very characteristic aspect of (Enothera lata has been 

 ascribed to its single supernumerary chromosome. Miss 

 Lutz has shown that many mutations with 15 'chromo- 

 somes do not have at all the characteristic lata appear- 

 ance, which must therefore be attributed to the duplica- 

 tion of a particular chromosome, rather than of any 

 chromosome. 



The production of mutations with irregular chromo- 

 some numbers is not confined to (Enothera Lamarckiana. 

 Two other species have given rise to mutations with 28 

 chromosomes, and in one case, that of (E. stenomeres, the 

 gigas mutation is entirely comparable in its characters 

 with (E. gigas de Vries. Its wood structure has been 

 compared with that of the parent species and has been 

 found to present deviations as great as those which are 

 apparent in the external aspects of the two plants. The 

 differences concern not only the relative size of the ele- 

 ments, but also their shape, and, to a certain extent, their 

 distribution. In typical (E. stenomeres the medullary 

 rays arc sometimes 141) cells high, whereas in mutation 

 gigas they are typically less than 25 cells high, and as 

 far as we have observed, never over 50. It is very sig- 

 nificant indeed that striking structural alterations in the 

 most conservative tissues of a plant may be instituted by 

 a single mutative evolutionary step. 



As far as the mutations with modified chromosome 

 numbers are concerned, there is the best of evidence that 

 the processes of mutation and Mendelian segregation are 

 absolutely distinct and independent. The evidence is not 

 onlycytological, but also genetical, for no mutations of this 

 class show Mendelian inheritance when crossed with their 

 parent forms. Their significance in evolution is illus- 

 trated by many widely separated groups of plants in 



