254 EFFECTS OF EXPOSING GERM-CELLS TO RAYS OF RADIUM 



reality., a hybrid in which the unlike characters separated out in the 

 course of the cell-divisions involved in the formation of a bud.* 



The nature of any germ-cell may be altered in one of three ways : 



1. By the acquisition of one or more new factors not previously 

 present. 



2. By the change of any factor from the domination to the reces- 

 sive condition, or from the recessive to the dominant. 



3. By the complete loss of one or more factors. 



It is hardly probable, reasoning from other known facts, that the 

 acquisition of a new factor could be accomplished by exposure to the 

 radium rays, but it is quite conceivable that, by such treatment, a 

 factor might be changed from the dominant to the recessive condition, 

 or that a complete loss of a factor might result. Shull's '^ experi- 

 ments with hybrid beans led him to suggest the hypothesis that unit 

 characters are determined by the simultaneous action of two or more 

 dominant factors or units in the germ-cells, and " that the later 

 specific or varietal derivatives were produced by the disappearance 

 of one or more of these original units as a dominant characteristic." 

 Thus, if the original character is determined by the dominant units 

 ABCDEFGH, "the later derivatives may be ABCDEFGh, 

 ABCDEFgH, ABCDE/gH, etc., through all the possible permuta- 

 tions . . . This conception results in an interesting paradox, namely, 

 the production of a new character by the loss of an old unit." 



This hypothesis seems to offer a plausible explanation of the pos- 

 sible induction of mutation by exposing either one or both of two 

 conjugating gametes to the rays of radium : and furthermore it dis- 

 closes a possible mechanism, such as is demanded by the theory that 

 the morphologically asymmetrical plant (figure 71) and the specimen 

 with two shoot-systems (figures 72 and 73), though falling under the 

 head of bud-sports, are fundamentally hybrids. Here also lies the 

 warrant for at least one interpretation of the significance of bud- 

 sporting, in general, as pointing to the fundamentally hybrid nature 

 of the organism thus sporting. This, by no means, excludes the other 

 interpretation of a bud-sport as a mutation, pure and simple, taking 

 place in the somatic cells during the formation of a bud, rather than 

 at some stage in the formation of the germ-cells. 



*This discussion was written in December, 1907, before the appearance of East's^ 

 paper on bud-variations (April, 1908). Metcalf ^^ has also recognized that bud-variation 

 and seed-variation are practically identical. 



