206 GENERAL BOTANY 



SUMMARY 



Variations. Variations form the materials from which new and 

 improved races of plants are built up, either in wild nature or in 

 culture. These variations arise in three ways ; namely, by crossing 

 and hybridization, by induced or spontaneous fluctuating variations, 

 and by mutation. 



Selection is the method employed in all cases for the isolation 

 of a new race, or for the gradual accumulation of variations in the 

 direction desired by the cultivator. 



Selection and hybridization. The variations due to hybridizations 

 and to crossings usually arise from the combination and reshuffling 

 of characters which originally existed in the parents of a cross. The 

 offspring produced by a cross split in succeeding generations, in 

 accordance with Mendel's laws, into a great variety of forms, in which 

 the parental characters are differently combined. Out of the variety 

 thus produced new and improved races can be selected and multiplied 

 by pedigree culture. 



Selection of fluctuating variations. The improvement of plants 

 by the continuous selection of forms which vary in the desired 

 direction is one of the oldest methods of securing improved races 

 of cultivated plants. " Nature gives successive variations ; man 

 adds them up in certain directions useful to himself. . . . The key is 

 man's power of accumulative selection." Darwin thus expressed 

 the essential facts relating to the improvement of plants by the 

 selection of fluctuating variations. The points of disadvantage in 

 the method are, first, that races and varieties thus produced are 

 unstable, tending to run out when selection ceases ; and, secondly, 

 that they are limited in the degree to which they can be improved 

 by selection. 



Selection of mutations and pedigree culture. Mutations are varia- 

 tions in one or more characteristics of a plant which arise suddenly 

 and are stable from the beginning. These mutations are either 

 small, creating the impression of a mere variation in leaf, flower, or 

 fruit, or they may be large enough to produce at once a new kind 

 of plant, which will be recognized by botanists as a new variety or 

 small species, now called an elementary species. The recognition 

 and selection of mutations, especially those large enough to consti- 

 tute a new elementary species, is one of the surest and best methods 

 for securing an improvement in any variety of plant. The reason 



