MAIZE BREEDING 



201 



1. Immediate effect of crossing on size of seed. 



2. FI varietal crosses. 



3. Isolation of homozygous strains. 



Immediate Effect of Crossing on Size of Seed. The question 

 of immediate effect of crossing on size of seed has received con- 

 siderable attention, and Carrier (1919) has recently considered 

 this a main cause for the conflicting results of corn experiments. 

 He demonstrated the fact that mixtures of seed of different strains 

 gave higher yields than seed of a single strain and explained the 

 results on the basis of increased yield due to the increased weight 

 of the endosperm of varietal crosses as compared with normally 

 pollinated seeds within a variety. 



Other investigations have partially supported Carrier's con- 

 tentions. Studies of the effect of pollen of a different strain or 

 variety on endosperm development are given in Table XLVIII. 



TABLE XLVIII. IMMEDIATE EFFECT OF POLLINATION ON ENDOSPERM 



WEIGHT 



These results show that there was an immediate effect of pollen 

 on the weight of the endosperm of crossed seed compared with 

 that produced by intra-varietal pollination. In varietal tests, 

 however, as conducted by the plot method, the degree of crossing 

 between different varieties would not usually be over 50 per cent. 

 Averaging the results of Wolf and of Collins and Kempton gives 

 about 5 per cent, increase due to crossing. Reducing this by 

 half gives an error in varietal tests of not more than 2.5 per cent, 

 as a result of increased endosperm development due to the im- 

 mediate effect of foreign pollen. As the studies of Collins and 



