No. 643] 



GROWTH IN CHROMOSOMES 



171 



are markedly influenced by the vigor of the plants, there 

 is a reduction in variability. Particularly is this true 

 of length of ear and weight of grain, whkh are fairly 

 reliable measures of the plant's reproductive ability, 

 which in annual plants sums up the organism's entire 

 energy. In other words the plants are uniformly vigor- 

 ous and are not dependent upon exceptional individuals 

 for their high average position. This is indirect evidence 

 that those hereditary factors which are concerned with 

 the growth of the plants are numerous and widely dis- 

 tributed throughout all or many of the chromosomes. 



As a means of corn improvement it would be highly 

 desirable to bring together into a pure breeding homo- 

 zygous condition all those factors which cause the hybrid 

 plants to excel their parents. Such individuals should 

 be even more efficient in their growth processes than the 

 heterozygous combinations of the same factors because 

 the determiners responsible for hybrid vigor seldom show 

 complete dominance. The recombination of linked fac- 

 tors is a problem that demands the most careful attention 

 of the plant and animal breeder. It is the closely linked 

 factors which are the main concern. When the distance 

 between any two loci is fifty units or more, then all the 

 factors situated outside of these points are independent 

 of each other in transmission and it makes no difference 

 from the standpoint of recombination whether the factors 

 are in the same or different chromosomes. Therefore 

 the number and arrangement of the individual genes 

 themselves seem to be more important than the number 

 of chromosomes. Although as yet it is impossible to com- 

 pare the numbers of factors in different species, it 

 does not seem likely that the genus Rosa with 8 chromo- 

 some pairs is genetically less complex than Nicotiana 

 species with 24 pairs. Some crustacean species with 

 84 pairs of chromosomes are contrasted with various 

 mammals with 8 to 12. Even in the Arthropods alone the 

 haploid number ranges from 2 to 100. It seems profitless 

 to look for any significance in chromosome, numbers. 

 Leaving aside the matter of doubling of chromosomes any 



