EFFECTS OF EXPOSING GERM-CELLS TO RAYS OF RADIUM 245 



the reducing mitosis in the sporogenesis of the mature zygote, it is 

 not difficult to imagine how a plant with two unlike sides might result 

 from altering the nature of either chromatin-mass. 



In the normal production of an elementary form by mutation, the 

 mutation is believed by de Vries,^^ to be " decided within the seeds." 

 There is no experimental evidence, however, for not considering 

 that, in sexual reproduction, the change may occur at any point in the 

 life-cycle of the germ-plasm, at least from gametophyte mother-cell 

 on, in either the maternal or the paternal line, or in both. 



We may conceive of the morphologically asymmetrical plant as 

 the result of some such sequence of events as follows : 



1. A destruction (or change from a dominant to a recessive con- 

 dition) by the radium rays, of the factor in the exposed pollen essen- 

 tial for the production of the biennis type of leaf. 



2. By the fertilization of a presumably normal biennis egg by a 

 sperm-cell from this pollen, an oosperm may result containing fac- 

 tors representing different peculiarities of leaf-form from each 

 parent. 



3. A unilateral expression of these peculiarities in the resulting 

 plant. This step would result from a division of the fertilized Q.gg in 

 its first and subsequent mitoses in such a way as to confine the mater- 

 nal chromatin to one side of the organism and the paternal chromatin 

 to the opposite side. This would offer a reasonable explanation, 

 also, of the transitional leaves between the opposite halves, for the 

 primordia of these leaves are probably composed of adjacent cells 

 from each side of the plant, thus giving rise to the observed bilateral 

 asymmetry of the leaves. The plant, then, so far as this one fea- 

 ture is concerned, is really a hybrid between two elementary forms. 

 The leaf-character of one of the parents has formerly existed only 

 potentially in the male germ-cell, and finds morphological expres- 

 sion for the first time in the offspring of the first generation. 



In " The Variation of Animals and Plants under Domestication," 

 Darwin^ records the testimony of Salter who, he says, " informs me 

 that at first a branch often produces variegated leaves on one side 

 alone, and that the leaves are marked only with an irregular edging 

 or with a few lines of white or yellow. To improve and fix such 

 varieties he finds it necessary to encourage the buds at the bases of 

 the most distinctly marked leaves, and to propagate from them 

 alone." 



