HEREDITY AND VARIATION IN PLANTS 143 



Hybridizing, and plant breeding generally, depend on the 

 laws of heredity for their success. It is a force of stabiliza- 

 tion which would result in all organisms being exactly alike 

 if variation did not take place too. If all organisms were 

 alike, crossing would have no value. 



Variation is the tendency for all organisms to be different. 

 It is the exact opposite of the tendency of heredity. Vari- 

 ation may be due to factors within the organism or to the 

 environment. Evidence of variation is all around us; no two 

 leaves are exactly alike, flowers are all different, and the 

 Government uses the Bertillon system of identification be- 

 cause no two human beings have the same fingerprints. 



Variations may be divided into two classes: first, due to 

 the combination of two different sets of genes at the time of 

 fertilization, resulting in different groups of characters in 

 each individual. No two children of a family are alike or 

 look alike, hence they show variation due to different combi- 

 nations of genes. Variations caused by crossing will breed 

 true, but it must be remembered that the Mendelian laws 

 will be obeyed for many of the characteristics. The vast 

 possibilities for combinations of chromosomes is a fruitful 

 field for variations. 



. In an unknown number of cases, variations are caused by 

 the mixing of the chromosomes. The one most studied is the 

 cross-over, where the homologous chromosomes exchange a 

 portion of their length. Some of the genes of a chromosome 

 from the male parent will take the place of the homologous 

 genes in the chromosome from the female parent. Cases of 

 cross-over are discovered by studying the transmission of 

 characters known to be on the same chromosome. 



A mutation is a second class of internal variations. A 

 mutation is sudden and breeds true. It appears to be a 

 change in a gene, brought about by some external influence. 

 Often a bud grows into a different kind of branch, or bears 

 different flowers or fruit. Thousands of bud mutations have 



