220 HEREDITARY TRANSMISSION [CH. 



the nuclei at some stage, or the chromosome must undergo a 

 qualitative change during the development of the plant, 

 and it should be noticed that in the "rogue" pea the change 

 of type during the growth of the plant is a constant pheno- 

 menon in some varieties, and not the apparently irregular 

 and accidental event to which bud variation is commonly 

 ascribed. Until cases such as these have received an ade- 

 quate explanation, the hypothesis that the segregation of 

 Mendelian characters takes place by the separation of 

 chromosomes at the maturation divisions of the germ-cells 

 cannot be regarded as fully established. 



A further difficulty of somewhat the same sort arises, 

 especially in plants, from the divergence from expectation 

 of the numbers of germ-cells bearing thejdternative charac- 

 ters of a pair. If in the production of germ-cells by a hetero- 

 zygote the segregation of alternative characters takes place 

 at the maturation divisions, there should always be equal 

 numbers bearing each of the two characters. When the 

 numbers from several or many families are added together, 

 this is generally true, except when disturbing causes, such 

 as differential mortality, intervene, but especially in plants 

 it is often not true of individual families. Quite often in some 

 families there is a constant excess of one character and 

 deficiency of the other, as compared with expectation, and 

 this has led some investigators to believe that the segrega- 

 tion occurs at an earlier stage, and that in some individuals 

 an excess of cells bearing one character, in others of cells 

 bearing the other, is produced by unequal multiplication of 

 the two kinds of cells in the germinal tissue. Such multipli- 

 cation could only occur before the maturation divisions, for 

 in the higher plants no multiplication of germ-cells occurs 

 after it. Irregularities of this sort, while not unknown in 

 animals, are less frequent in them than in plants. 



