SOME PRINCIPLES OF PLANT BREEDING 501 



other desirable qualities. The cereals for example must be prop- 

 agated from the seed. 



672. Selection and fixing of varieties for special pur- 

 poses. Varieties are selected and propagated for various pur- 

 poses, as for color of flour, high percentage of gluten, or oil, or 

 starch, etc. Sugar beets are selected for productivity and high 

 percentage of sugar. It is noteworthy that in all cases when a 

 variety has been bred to a high state of excellence, continued 

 selection of seed from the best must be followed up continuously, 

 otherwise the variety will deteriorate, since even with the best and 

 most highly bred varieties variation continues. The variety is 

 not fixed by selection so that it cannot change. By selection 

 man can intensify the desired character and increase the fre- 

 quency with which it occurs. He can do no more. If selection 

 were not continued, the varieties would gradually deteriorate 

 since in the case of those where cross pollination took place the 

 variety would gradually sink to the level of the average. This 

 average would give varieties still lower than the lowest of the 

 highly bred variety which would tend to a still lower average 

 level. The same result would also finally be reached in the case 

 of those varieties which were self-pollinated if selection were not 

 continued. Selection should also be made in the light of corre- 

 lated variation. For example, the selection of the largest ears of 

 corn which could be found in a field would not necessarily give 

 the best results. The largest ears borne on the best stalks, and in 

 some cases on stalks where there is more than one ear, give better 

 results; also ears with the highest percentage of grain in pro- 

 portion to cob. But recent experiments show that the best 

 method is selection from an area which gives the highest yield 

 per acre. In the case of plants which can be propagated asex- 

 ually, variation in the desired variety can be lessened and the 

 character more permanently fixed by asexual methods of propa- 

 gation,* as by grafting in the case of apples, pears, etc., by 



* This is sometimes called asexual multiplication of extremes. When a 

 variety from which it is desired to select has reached a high state of fluctua- 

 ting variation, it is the extremes of variation which are selected, and usually 



