200 Plant Genetics 



which chromosomes are composed the male contributes 

 only chromosomes to the progeny, and judging from 

 breeding results the female contributes nothing more. 

 Thus by elimination the conclusion is reached that 

 chromosomes are the bearers of hereditary characters. 



(4) In addition to the foregoing arguments there 

 are some minor cytological evidences that confirm the 

 chromosome theory. Tetraploid sporophytes, that is, 

 sporophytes with twice the normal number of chromo- 

 somes, have appeared as mutants, as in Oenothera, or in 

 apospory, as in Marchal's mosses; such sporophytes 

 are distinctly larger and more vigorous than their 

 normal ancestors. This is indirect confirmation of the 

 chromosome hypothesis. Many hybrids are sterile, and 

 in some cases cytology shows that this sterility is the 

 result of, or at least associated with, a chaotic condition 

 of the chromosomes in the germinal tissue of the hybrid, 

 while everything else in the hybrid seems normal. 



Breeding. — (i) Mendel's theory of segregation, 

 which must be regarded as well estabhshed, requires that 

 the bearers of hereditary characters must behave in a 

 certain particular way at maturation of the gametes. 

 This necessary behavior is found to occur quite regularly 

 among the chromosomes at the reduction division, and 

 nothing but chromosomes can be shown to behave in 

 this way. 



(2) Mention has been made of Morgan's unique 

 work on linkage and crossing over. It is only necessary 

 to say that his unusual results are associated with 

 observed irregularities in the behavior of chromosomes, 

 and furnish strong confirmation of the chromosome 

 hypothesis. 



