338 ELEMENTARY STUDIES IN BOTANY 



it has been found that this pure strain deteriorates to a cer- 

 tain level ; in other words, it is not so good for our purposes 

 as the mixed races. In this case it is found that if two pure 

 strains are crossed, the resulting hybrids are more vigorous 

 than either parent. The ideal corn-breeding, therefore, is 

 rather a complex operation, involving two distinct operations : 

 (1) pedigree culture, which separates pure strains from a 

 mixture; (2) combination of pure strains, which secures 

 vigorous plants. The question might be asked, why is it 

 desirable to separate pure strains from mixtures, only to 

 combine them again in new mixtures? The answer is that 

 unless the pure strains are separated and recombined, the 

 mixtures are chance mixtures rather than intelligent mixtures. 

 There are very many people, however, who prefer to use 

 chance rather than intelligence. 



43. Hybridization. In the operations of plant-breeding 

 described above, reference was made to " crossing " ; that is, 

 the use of two parent plants of different kinds, resulting in 

 what is called a hybrid. Hybridization is a very important 

 operation in plant-breeding, for by means of it certain desir- 

 able qualities that are separated in two kinds of plants may be 

 combined in a single individual. It must not be supposed 

 that the desired combination will appear in all of the hybrid 

 progeny. It is only when thousands of hybrids are produced 

 that there is any certainty that the desired combination will 

 be found among them. But when it is found, the plant 

 possessing it can be pedigreed and multiplied. 



There is a limitation in the use of hybrids that must be 

 noted. A hybrid combines characters of two parents, and 

 when it produces progeny by means of seeds, only about one- 

 half of the new individuals will continue the combination, 

 that is, will continue to be hybrids. The other half will 

 resemble one or the other parent. This is what is called 

 the " splitting " of hybrids, and they split up in a definite 

 ratio, which is called Mendel's law, and this same ratio of 



