BOTANY 



fundamental number is fourteen, or seven pairs. 

 But mutations were found having fifteen, twenty- 

 one, and twenty-eight, as well as other numbers. 

 How these new numbers arise we have not time to 

 consider, except to say that different processes are 

 involved. But having arisen, a given number is 

 characteristic of every cell of the new form. In 

 CEnothera lata there are always fifteen chromo- 

 somes in the nuclei, in whatever part of the plant 

 the chromosomes are counted. These and other 

 facts, such as the doubled number (twenty-eight) 

 of chromosomes in CE. gzgas, led to the point of view 

 that each mutation is a cell-change, originally 

 happening in the nucleus of a particular germ-cell 

 and handed on from one cell generation to another 

 by the mitotic mechanism. 



But only a certain number of the mutations 

 of CEnothera show a visible change in the struc- 

 ture of their nuclei. These exhibit peculiar types 

 of hereditary behaviour depending on how their 

 extra chromosomes are distributed in the germ 

 nuclei. In other mutations, such as brevistylts 

 and rubric -a/yx, there is no visible change in the 

 structure of the nuclei, and the inheritance of the 

 new character follows the simple Mendelian rule. 

 In such cases it appears that a change, probably 

 of a chemical nature, has occurred in one portion 

 of a particular chromosome. We thus arrive at 

 a means of accounting for not only the manner of 



173 



