INBREEDING EXPERIMENTS 127 



view of the further reduction in other strains, it would 

 have been extremely diliicult. {Since plants are frequently 

 produced which cannot be perpetuated, however, it is 

 to be expected that some strains will also be found 

 which cannot survive. This is good evidence that strains, 

 differing markedly in their ability to grow, are isolated 

 by inbreeding. 



Plants of the surviving strains, while smaller in size 

 and lower in productiveness, are perfectly healthy and 

 functionally normal in every way except that in many of 

 them there is an extreme reduction in the amount of pollen 

 produced. These infertile types are dependent on other 

 plants for pollen in order to make the yields they show 

 in open field culture ; when grown by themselves the yield 

 is less due to an inadequate supply of pollen. On the other 

 hand, this extreme reduction in pollen production is not 

 shown by all the strains, some inbred strains producing 

 pollen abundantly. 



Prom the data given in Table III there is considerable 

 evidence that these plants have reached about the limit 

 of their reduction in size and productiveness and tluit 

 whatever changes have taken place in the last three years 

 have been slight. Further inbreeding is necessary for 

 one to be positive on this point. But as the crosses within 

 these inbred strains have given no significant increases 

 over the selfed lines, and as there has been no visible 

 change in morphological characters, in the past three 

 years at least, it seems apparent tliat the reduction in 

 vegetative vigor and productiveness is ver>' nearly, if not 

 quite, at an end. 



Reduction is shown by inbred maize plants in other 

 characters. Length of ear, as well as height of plant and 



