248 



Emerson. 



On the basis of the soiiiatic-mutation hypothesis, all of these things 

 might happen, but the chance of their happening is very small ami the 

 possibility of being able to demonstrate their true nature, if they do 

 occur, is still smaller. 



A wholly colored or wholly starchy seed is occasionally found on 

 an ear of colorless sugar maize the pollination of which has been 

 guarded. Such a seed is commonly assumed to be due to the entrance 

 of a stray grain of pollen in the process of artificial pollination. This, 

 so far as tested, has been the true explanation in my own work and I 

 believe also in the work of other investigators. Such seeds have, so 

 far as tested, always shown the hybrid nature of the embryo, thus in- 

 dicating an accidental cross'). I do not recall ever having seen a half 

 starchy seed on a selfed, sugary ear. If such seeds had been observed 

 before my attention was called to this particular problem, I should 

 very likely have assumed that they were cases of accidental cross-polli- 

 nation and should probably not have tested the matter further. 



But progressive mutations are admitted to be very rare. The loss 

 of a character is commonly believed to be a much more fretjuent 

 occurrence than the acquisition of a new character. Should we not, 

 therefore, hope to find half-sugary seeds on homozygous, starchy ears? 

 A moment's consideration will convince us, I believe, that, granting its 

 possibility, the chance of such an occurrence is extremely small. 



In homozygous, starchy maize, the factor for starchiness, S, is 

 present in the endosperm in the triplex condition, SSS. Such evidence 

 as we have indicates that a mutation is unlikely to affect more than 

 one of the three S factors in such a cell. (See Emerson (11)). If 

 this be true, three distinct mutations would have to occur in a single 

 cell, or in a cell and its descendants, in order that a change from 

 starchiness to sugariness take place within the endosperm. East (8) 

 found one half-and-half seed in 10000. At the same rate, sugary endo- 

 sperm might be expected to appear in homozygous starchy material in 

 one seed on every one or two billion ears, allowing five hundred to 

 one thousand seeds per ear. 



It is quite as possible that the two half-and-half seeds described 

 earlier in this paper may have been due to somatic mutations as that 

 they may have resulted from somatic segregations. And, if the latter, 



') Of course the embryo would be hybrid also if a mutation occurred prior to 

 the divisions separating the egg cell from the polar cells. 



