CHAPTER XL 

 PLANT BREEDING 



478. Definition of plant breeding. The selection and mainte- 

 nance of the most desirable varieties of cultivated plants must, 

 to some extent, have occupied the attention of agriculturists 

 during all the thousands of years since farming began. 1 From 

 the writings of Virgil and other Latin authors it is clear that 

 Roman farmers practiced careful selection of cereals for seed, 

 knowing that without this their crops would diminish. But it 

 is only within a short period that scientific principles have been 

 brought to bear on the process. In fact, it is stated that the 

 systematic improvement of races of cultivated plants began in 

 the middle of the nineteenth century. The intentional produc- 

 tion and perpetuation of new varieties is known as plant 

 breeding. It is based upon the methods outlined in Sees. 479 

 and 480. 



479. Single selection and continued selection. New varie- 

 ties of plants, whether wild or cultivated, are constantly being 

 produced by ordinary variation and by mutation (Sees. 472-474). 

 In a single field, supposed to contain only one kind of wheat, a 

 trained botanist once found twenty-three well-marked varieties, 

 one of which became the parent of a sort that has remained famous 

 for over three quarters of a century. The plant breeder is con- 

 stantly on the watch for promising varieties, preserving all which 

 seem likely to be of use. While it is a slow, uncertain method 

 to await the appearance of variations in any desired direction, 

 and then to rely on the perpetuation of these, the large num- 

 ber of valuable new varieties thus secured warrants all growers 



1 In China the cultivation of rice, wheat, two kinds of millet, and soy 

 beans dates back at least 4600 years. 



500 



