PLANT BREEDING 517 



can be propagated vegetatively are also for the most part the 

 results of hybridization. Occasionally a new variety is based on 

 a chance mutation which continues to breed true. The Dutch 

 botanist Hugo de Vries obtained eight different mutations of 

 evening primrose, as well as others in various species of plants. 

 Through these methods it is now possible to improve the 

 quality of plants ; for example, quality of cotton, quick ripening, 

 resistance to disease, and so on. It is possible also to make 

 special breeds for particular localities that are suitable only for 

 plants with special combinations of characters. 



PLANT BREEDING 



1. Induced variations in plants 



Why variation is desired (increased chance of finding more use- 

 ful types) 

 How variations are induced 



Maintaining best living conditions for plants 



Expectation that all possibilities will come out 

 Changing conditions in detail 



Expectation that individuals will develop new characters 

 Hybridization (crossing individuals that differ) 



Expectation that offspring will show new combinations of 

 characters or extreme degrees of some characters 



2. Instability of offspring from induced variation 



Fluctuations, depending on the conditions, do not reproduce 



themselves 

 Hybrids "split up" or "throw back" to ancestral types 



3. Preservation of new variations by vegetative methods 



Propagation 



Tuber ; bulb ; cuttings ; layering 

 Grafting 



4. Laws of heredity (Mendel's principles) 



Dominance 

 Segregation 

 Combination (unit characters) 



5. Practical breeding 



Find variations or mutations and see whether or not they breed 

 true 



