PLANT BREEDING AND EVOLUTION 



197 



accumulation of variations are limited in the degree to which 

 they can be improved. Thus, the sugar beet has been cultivated 

 and selected for a hundred years or more, but it has not been 

 found possible to increase the average sugar production of a 

 selected field of beets beyond from 16 to 18 per cent. The Illi- 

 nois experiment station has had a similar experience in attempt- 

 ing to raise the percentage of oil and protein in corn beyond 

 the high percentages mentioned above. A similar condition 

 seems to prevail among all cultivated varieties, where ordinary 

 variations are used as a basis for selection. 



Two KINDS OF VARIATIONS 



Two kinds of variations are now recognized by scientists 

 and plant breeders, known respectively as fluctuating (or con- 

 tinuous) variations, and mutations (or discontinuous variations). 

 .Fluctuating variations 

 are the ordinary dif- 

 ferences observed in 

 plants of the same 

 kind in a field or a 

 garden. They are due 

 to the environment, 

 such as rich or poor 

 soil, abundance or 

 lack of water, sun ex- 

 posure or shade, and 

 many other environ- 

 mental influences to 

 which plants growing 

 in the open are sub- 

 jected. They are fur- 

 thermore inconstant, 

 not tending to be perpetuated in the offspring of plants mani- 

 festing them. We all know, for instance, that seeds from a 

 large, healthy tomato plant, produced by rich soil and careful 

 culture in a garden, will not bear similar offspring unless they 



FIG. 107. Connecticut broad-leaf tobacco, 

 unselected 



Showing variation in tobacco due to change of soil 



and climate. Photograph furnished by Connecticut 



Agricultural Experiment Station 



