482 TEXTBOOK OF BOTANY 



of plants, such as wheat and rye, wheat and couch grass, radish and 

 cabbage. Here again there is often a high degree of sterihty and many 

 pecuhar irregularities are obtained. Radish and cabbage each have 9 

 pairs of chromosomes and their Fi hybrid has 9 cabbage chromosomes 

 and 9 radish chromosomes. Among the F2 hybrid segregates was a hybrid 

 containing 18 cabbage chromosomes and 18 radish chromosomes, which 

 must have been obtained from gametes formed without reduction divi- 

 sion. Subsequent generations of this particular hybrid also contained 

 9 pairs of radish chromosomes and 9 pairs of cabbage chromosomes. 

 Apparently the cabbage and radish chromosomes did not pair with each 

 other during reduction division. This new hybrid has all the character- 

 istics of a distinct species. Perhaps species are sometimes formed in this 

 way in the wild state. 



Influence of environment. The above summary of some of the better- 

 known types of aberrations in chromosomes and genes is barely more 

 than an introduction to this particular phase of natural phenomena. 

 But perhaps it is sufficient to indicate some of the research that is being 

 done and to supply a basis of understanding as to how hereditary dif- 

 ferences originate. All such diflFerences begin as changes in the composi- 

 tion and arrangement of units of matter in the protoplasm of cells. For 

 example, the molecules of which genes are composed may undergo 

 change in composition. We have already seen that changes in the com- 

 position of molecules may be the result of a change in the arrangement 

 of atoms, or of a loss or gain of certain atoms. In an earlier chapter it 

 was shown that a rearrangement of a few atoms in a molecule of glucose 

 resulted in the formation of fructose, and vice versa; and that the com- 

 position of molecules is altered by oxidation-reduction processes. 



Next in order are changes in the arrangement of genes within the 

 chromosomes; and among the larger units are changes in the arrange- 

 ment of chromosomes themselves. Changes in the composition of the 

 molecules of the genes are often regarded as the most fundamental 

 ones. They are chemical changes, while all the others are physical 

 changes. Each of these changes is the first step in a type of heritable 

 variation known as a mutation. In brief, a mutation may occur among 

 the chromosomes as a whole, among the genes within a chromosome, or 

 among the molecules and atoms within a gene. 



Moreover, mutations may occur in an apparently uniform environ- 

 ment, or the same kinds of changes may occur in different kinds of 

 environments, and their frequency of occurrence may be increased or 



