Plant-Breeding for Farmers. 141 



breeders in the case of plants which normally cross-fertilize, in planting 

 continuously 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 with careful 

 selection, 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 developed in the careful 

 breeding and improvement of corn. The corn plant has been shown, as 

 a result of experiments carried on by various investigators, as, for 

 instance, the Illinois Experiment Station and by the U. S. Department of 

 Agriculture, to lose vitality very rapidly when self-fertilized. Within 

 three or four generations by careful self-fertilization it is possible to pro- 

 duce a strain of corn of almost total sterility. The general practice of 

 corn-breeders who have been giving attention to the production of 

 highly bred 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, further 

 plants to continue 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 find- 

 ing in some cases an apparent decrease in yield, which seems to be 

 traceable to the eft'ect of inbreeding. It, therefore, seems necessary for 

 us here and in other plants that are eft'ected by inbreeding to devise 

 some methods that will avoid close inbreeding, Alethods for the use 

 of corn-breeders will be described later in this bulletin. The detri- 

 mental eft'ect of inbreeding is largely limited to those plants which 

 are normally cross, fertilized, this fact being strikingly brought out in 

 Darwin's famous "Investigations on Cross- and Self-fertilization in the 

 Vegetable Kingdom."' Tobacco, wheat, and some other plants which are 

 normally self-fertilized do not show this decrease in vigor as a result of 

 inbreeding. Indeed, in such plants, cross-fertilization ordinarily results in 

 decreased vigor and should be avoided. 



Obviously in the case of clonal-breeding, such as the improvement of 

 potatoes by hill selection, the isolation of the breeding stock does not have 

 to be considered and the breeding and increase patches can be planted with 

 the general crops if so desired. 



