12 CIRCULAE NO. 124, BUREAU OF PLANT INDUSTRY. 



VOLUME AND SPECIFIC GRAVITY OF SEEDS. 



The differences in weight observed in these seeds might come about 

 through differences in size or tlirough differences in composition, or 

 a combination of both. To determine this point, vohimetric and 

 specific-gravity determinations wore made of the pure and liyl)rid 

 seed of all tiie ears. The results showed that the observed differ- 

 ences in weight were associated with corresponding differences in 

 volume. No differences in the specific gravity of pure and hybrid 

 seed from the same ear could be detected. 



SIZE AND COLOR NOT CORRELATED. 



In order to distinguish between pure-bred and hybrid seed on the 

 same ear only those hybrids with colored aleurone could be utihzed. 

 It might be urged that there was a tendency for colored seeds to be 

 heavier than white and that we liave been measuring differences 

 between colored and white seed rather than dift'erences between 

 pure and hybrid seed. Fortunately, there was material at hand to 

 test this point. In 1911 a cross had been made between Variegated 

 and Mexico Black, two of the varieties used in these experiments. 

 This hybrid was grown and a number of self-pollinated ears secured 

 in 1912. These ears all had both white and colored seeds. A com- 

 parison of the weight of the white and colored seed from each of 

 these ears showed only the ordinary fluctuating differences between 

 the two classes. 



SELECTIVE POLLINATION AND CROWDING OF SEED. 



Before accepting the increased size of the hybrid kernels on mixed 

 ears as an indication of the increased yields that may be secured 

 from cross-polhnation, it will be necessary to consider the possibility 

 that the hybrid kernels might develop at the expense of the neigh- 

 boring pure kernels. In that case the average size of the seed would 

 not be increased if all the seeds of the ear were cross-j)oUinated. 



If the pure kernels are weak or develop more slowly, the hybrid 

 kernels niay gain by being less crowded, or the hybrid kernels may 

 grow more rapidly at first because of more prompt poUination and 

 may then rob their pure-seed neighbors b}^ direct appropriation of a 

 larger share of the available food materials. 



A fortunate accident in pollination tlu'ows light on both of these 

 questions. An ear of "Maryland White Dent" was polHnated by 

 a plant of the same variety. Seven days later, in making a second 

 a])])lication of pollen, a plant belonging to a red variety with yellow 

 endosperm was accidentally used as the source of pollen, tlie mis- 

 take being noted at the time. 



[Cir. 124] 



