152 Outline of Genetics 



could contribute no color. The remainder of the endosperm, 

 produced by the male nucleus (either (i) independently or (2) 

 in conjunction with one of thepolars), would, of course, show the 

 red aleurone characteristic of the male parent. 



The arrangement of the red and white areas, sometimes accord- 

 ing to the mottling plan and sometimes according to the mosaic, 

 might be explained by the usual method of endosperm formation. 

 Endosperm formation begins with free nuclear division, the result- 

 ing nuclei being free in the cytoplasm of the embryo sac. The 

 cell walls are not formed for some time; sometimes not until 

 nuclear division is completed. Before a large number of free 

 nuclei have appeared they move from the central region of the sac 

 and usually become placed near the wall, where free nuclear divi- 

 sion continues. When walls begin to appear, separating the 

 nuclei, wall formation begins at the periphery of the sac and 

 extends toward the center, in what is called centripetal growth. 

 This program, which is common in seed plants and is known to 

 occur in wheat, is doubtless the program in corn. If, then, the 

 second male nucleus fails to unite with the fusion nucleus and each 

 divides separately, when their progeny nuclei move out to the 

 periphery of the sac the nuclei of male and female origin may 

 well become more or less mixed. In their further division, there 

 w^ould be groups of cells of male origin interspersed among groups 

 of female origin. The result would be red and white areas on 

 the mature grain, intermingled as irregular blotches, giving the 

 mottled effect (a). On the other hand, if the daughter-nuclei of 

 the male and female components migrated en bloc to the wall of 

 the embryo sac, and no mixing occurred between nuclei of the 

 tw^o types, the result would be the production of anomalous grains 

 of the mosaic type {b). 



These ingenious proposals of Webber's helped to focus the 

 attention of other investigators upon the problem imposed by the 

 occurrence of anomalous grains of these two types. 



Webber's conception of the mottled grains (a) was shown 

 to be fallacious by the experiments of Kempton (6) and of Emer- 

 son (4). It was found that mottled grains, instead of being 

 anomalies as Webber had believed, would appear in considerable 

 numbers and with dependable regularity under the proper con- 



