1^0. 643] 



GROWTH IN CHROMOSOMES 



169 



duction of grain (weight of entire pistillate inflorescence 

 with mature seeds). A previous study of a large number 

 of first generation hybrids between inbred strains of 

 maize has shown that the average number of rows of 

 grain of the hybrids was increased 5.29 per cent, above 

 the mean position of their parents; similarly nodes per 

 plant 6.45 per cent.; height of plant 27.44 per cent.; 

 length of ear 28.57 ; and total production of grain 180.00 

 per cent. The variability of these Fj plants was slightly 

 decreased below the parental average in nearly every 

 case in respect to these characters. 



Rows of grain and nodes are therefore much less in- 

 fluenced by the vigor of the plant than are the other 

 characters, no'tably production of seeds, wliieli is xevy 

 largely determined by the amount and ra])idity of growth. 

 Assuming that the complementary action of dominant 

 favorable growth factors is responsible for the vigorous 

 growth of the hybrids, it would be expected that X 

 combination would not be as variable as the second gen- 

 eration resulting from self-fertilization in respect to 

 production of grain per plant, provided a large number 

 of essential growth factors were acting and that these 

 were distributed rather uniformly throughout the chro- 

 mosomes. On the other hand such characters as rows 

 of grain on the ear and nodes per plant being largely 

 independent of growth vigor, would not be expected to 

 show a reduction in variability when compared with the 

 second self-fertilized generation. 



The distribution and statistical constants for the sec- 

 ond generations grown from self-fertilized seed of the 

 parental hybrids have been compared to the reciprocal 

 crosses of the same parental hybrids in three different 

 sets of plants. In each case the cross-fertilized seed, 

 which produced the Fj X F, plants, and the self-fertilized 

 seed, from which the Fg plants were grown, came from the 

 same ears. The two kinds of pollen were applied in a 

 mixture at one time and the seeds separated by their color 

 at maturity. 



Without giving the extensive data upon which the 



