3i6 CULTIVATED PLANTS AND THEIR ORIGIN 



5. Correlated variability. — The various parts of the body of 

 a plant or animal are so co-ordinated with each other that any 

 change in the structure or function of one organ very frequently 

 brings about some necessary change in another. The nature 

 of the connection between the correlated variations is in many 

 instances obscure ; nevertheless the existence of this kind of 

 variability must be always borne in mind by those who seek to 

 improve plants. Moreover, it is important that every endeavour 

 should be made to elucidate its nature, for a correct and 

 complete understanding of the structural and functional re- 

 lationships between the different parts of plants would enable 

 the plant-breeder to save much valuable time. There is little 

 doubt that through want of knowledge on such matters, plant- 

 breeders have not unfrequently attempted to do that which is 

 impossible. 



In a great many cases quantity of produce and good quality 

 are so connected that beyond a certain point the increase of one 

 brings with it a decrease of the other and to combine both 

 characters in one variety appears to be impossible. For 

 example, all attempts to obtain a race of sugar-beet with 

 very high yield of roots per acre and high sugar-content are 

 found to fail when a certain percentage of sugar in the root 

 is reached; with every increase of sugar-content beyond this 

 point there is invariably a decrease in size and weight of the 

 ' root' 



It appears to be impossible to breed a white wheat of rich 

 gluten-content, with as high a yielding power of grain per 

 acre as ' rivet ' starchy wheat ; this difficulty is partially de- 

 pendent on the fact that the glutinous albuminoids are largely 

 stored in the single layer of ' aleuron-cells ' which become 

 filled first, the central parts of the endosperm being filled up 

 later chiefly with starch; the longer the assimilation goes on 

 after the aleuron-layer is filled the more starchy the grain 

 becomes, and the larger the crop. 



