270 D. F. JONES. 



made for variations inherent in a problem of this kind the con- 

 clusion can be no other than that these plants manifest a definite 

 receptiveness to their own pollen, discriminating against foreign 

 pollen even though it comes from plants only slightly differen- 

 tiated from them, both of which might easily be descended from 

 the same individual at no very distant period back. This selec- 

 tive action is shown by plants of weak growth or full vigor, 

 whether each strain descended from a line of similar ancestors 

 or whether its immediate parents were diverse and, finally, 

 irrespective of the gametes being alike or unlike in germinal 

 contents. The one significant feature in common in all these 

 experiments is the fact that the cytoplasm which surrounds the 

 male nuclei and which makes up the vehicle that carries them to 

 the egg cells is alike for the gametes of any one type of plant 

 whether this plant is homozygous or heterozygous and in self- 

 fertilization this cytoplasm is the same as the medium in which 

 the pollen fulfills its function. This points very strongly to the 

 probability that the differential effect is due to the rate of 

 pollen-tube growth although it may be determined after the 

 male nuclei are brought to the egg. 



The average weight of the seeds in the different classes of all the 

 pollen mixtures is given in Table IV. with the increase in weight 

 of the cross-fertilized over the self-fertilized seeds. Expressed 

 as per cent, these figures permit an estimation of the comparative 

 amount of heterosis shown in the crosses. In fact this is one 

 of the best means of measuring the stimulation of heterozygosis 

 as the environmental differences are reduced to a minimum. 

 It has not been definitely proven that there exists a correlation 

 between the amount of heterosis in the seeds and that shown 

 by the resulting plants grown from those seeds but the indications 

 are that there is a close relation between the two. Since hybrid 

 vigor is roughly proportional to the germinal differences in the 

 two forms united it can be determined whether or not there is a 

 relation between the diversity of the plants used in the several 

 pollen mixtures and the degree of preference shown by those 

 plants to their own kind of pollen. Table V. shows that the 

 coefficient of correlation between the average increase in weight 

 of seeds and the deviation in favor of self-fertilization, both stated 



