60 



SOME OF THE PRINCIPLES OF PLANT- BREEDING 



that differs somewhat in its character but which 

 seems to be of exceptional value. The temptation 

 under such circumstances is to take this new plant 

 and discard the old ideal. Many breeders have 

 found that by taking such selections they have 

 made serious mistakes, and lost the improvement 

 already secured. Whenever a plant of different 

 character springs up it is entirely an unknown 

 quantity, and it may not transmit the desired 

 characters ; and, even if it should, they are differ- 

 ent from the qualities of the ideal strain for which 

 the selection was first started. 



Control of parentage. 



In plant-breeding, as in animal-breeding, the 

 isolation of the parents is a very important con- 

 sideration. It is necessary that we should know 

 the character of both parents whenever this is 

 possible. In breeding plants more attention is 

 given ordinarily to the mother parent, and in very 



Fig. 81. Loss of fertility in com by inbreeding. Pile on left 

 fertilized seed; on right from inbred or self- fertilized 



many instances the characters of the father parent 

 are entirely neglected. Animal-breeders, on the 

 contrary, give more attention to the characters of 

 the male parent, and much improvement in ordi- 

 nary herds has been accomplished by the introduc- 

 tion of improved blood through the male. In plant- 

 breeding, it is desirable that the seed of the select 

 individuals be planted in a field by themselves. 

 This insures that only progeny of caKefully selected 

 plants will be planted near together, and thus no 

 ordinary stock will enter as a contamination. One 

 can be certain that each plant of the progeny is 

 fertilized with pollen from another similarly good 

 plant, or at least from a plant derived from good 

 parentage. One difficulty, however, has been ex- 

 perienced by plant-breeders in planting continu- 

 ously their selected stock in such isolated plots. If 

 this method is continued year after year, it results 

 in fairly close inbreeding, which in the case of 

 plants frequently results in loss of vitality and 

 vigor. In animal-breeding it is apparently the 

 case that ordinarily there is no noticeable effect 

 from close inbreeding, and many of the most famous 

 animals have been produced as a result of the 

 closest in-and-inbreeding. In plants, however, it is 

 possible to secure much closer inbreeding than in 

 the case of animals, as in many cases a plant can 

 be fertilized with its own pollen. 



Within recent years much activity has been 

 shown in the careful breeding and improvement of 

 corn. The corn plant has been shown, as a result 

 of experiments made by various investigators, as, 

 for example, by the Illinois Experiment Station and 

 the United States Department of Agriculture, to 

 lose vitality very rapidly when self -fertilized. 

 (Fig. 81.) Within three or four generations, by 



the most careful inbreeding, it is possible to reduce 

 corn to almost total sterility. The general practice 

 of corn-breeders who have been giving attention 

 to the production of pedigree strains, is to plant 

 the rows of corn from different select ears side by 

 side, giving a row to each select ear, and each 

 year selecting, from the progeny of those rows 

 which give the largest yield, plants to continue 

 further the selection. Planting these select ears 

 together every year, therefore, means that they are 

 more or less inbred, as the closest relatives are 

 planted together in the same row. While in follow- 

 ing this policy at first no effect was visible, corn- 

 breeders are now finding in some cases an appar- 

 ent decrease in yield, which seems to be traceable 

 to the effect of inbreeding. It seems necessary for 

 us, therefore, in corn and in other plants that are 

 affected by inbreeding, to use methods that will 

 avoid close inbreeding. The detrimental effect of 

 inbreeding is largely limited to those plants which 

 are normally cross-fertilized, this fact 

 being strikingly brought out in Dar- 

 win's "Investigations on Cross- and 

 Self-fertilization in the Vegetable 

 Kingdom." Tobacco, wheat, and some 

 other plants that are normally self- 

 fertilized do not show this decrease 

 from cross- ^^ ^jg^j, ^ ^ result of inbreeding. In- 



^^ ' deed, in such plants cross-fertilization 



ordinarily results in decreased vigor and should be 

 avoided. 



Principles of selection. 



Selection is the principal factor of breeding, 

 both in the improvement of races and in the pro- 

 duction of new races or varieties. The keynote of 

 selection is the choice of the best, and a factor of 

 the highest importance is the examination of very 

 large numbers in order to secure the maximum. 

 Galton, writing on this subject, says : " One gene- 

 ration of 99-degree selection is seen to be more 

 effective than two generations of the 90-degree 

 selection, and to have about equal effect with the 

 the 80-degree selection, carried on to perpetuity. 

 Two generations of the 99-degree selection are 

 more effective than fpur of the 95-degree, and than 

 the perpetuity of the 90-degree." The use of de- 

 grees in representing the perfection in which a 

 character is shown may not be possible, but it is 

 possible for any breeder to examine large numbers 

 and to find one or two plants which produce in the 

 greatest degree the character desired. It is these 

 plants that should be preserved as mother plants 

 in starting the selection. 



In the production of new races, it is of interest 

 to us to know whether by pure selection we can 

 lead plants to vary so greatly that they may be 

 considered to have passed beyond the bounds of 

 the race, and thereby the breeder to have estab- 

 lished a new and distinct race. It is certain, of 

 course, that, by careful observation and selection 

 from any particular race, ultimately a new race 

 may be produced. The question is whether the 

 individual or individuals selected in producing the 

 new race have not varied by mutatian or seed- 



