272 GENETICS IN RELATION TO AGRICULTURE 



resulted in the production of some red pigment along with the yellow, 

 thus causing the narrow sector of deep orange chrome (Ridgway, No. 

 llh). That each of these changes occurred in a single cell is indicated 

 by the fact that the differently colored sectors are sharply defined through- 

 out and that the extremities of the orange red sector are extremely 

 narrow. J. B. S. Norton reports the origin of a color chimera in the Acme 

 tomato in which a branch of lighter green foliage appeared and the lighter 

 colored tissue could be traced down the stem to a point where it had ap- 

 parently originated in a single cell. Expanding as the stem grew, first a 

 portion of a leaf was involved and finally an entire bud was included, thus 

 giving rise to the sport branch. Undoubtedly this is the usual manner of 

 origin of natural chimeras. 



We have examined several typical cases of factor mutations in animals 

 and plants. From this evidence it is clear that factor mutations occur 

 in undifferentiated cells the germ cells in animals and either the germ 

 cells or any meristematic cell in plants. There is, of course, no a priori 

 reason why mutations should not occur in the somatic cells of animals. 

 A fairly common meristic variation is the reduplication of repeated parts 

 and it is possible that this departure from normal development is con- 

 ditioned by a factor mutation. The discovery of a germinal mutation 

 causing reduplication in animals would support this idea. Such a 

 mutation has been discovered by Miss Hoge who reported a recessive 

 factor for reduplication of the legs in the Drosophila. This possibility 

 of somatic factor mutations in animals has little practical significance on 

 account of the impossibility of propagating domestic animals asexually. 

 It has considerable theoretical interest, however, in its possible bearing 

 on the origin of certain diseases such as cancer. 



Vegetative Mutation Versus Somatic Segregation. Since the ma- 

 jority of bud sports are characterized by the replacement of a dominant 

 with a recessive character, it is not strange that both bud sports and 

 chimeras have been generally considered as due to "somatic segregation" 

 in heterozygous individuals. It is not yet known whether bud sports 

 occur more frequently in heterozygous than in homozygous individuals. 

 But this consideration is of less importance than the fact that somatic 

 factor mutations do occur, which seems to be well established. To 

 mention an illustrative case, Emerson has shown that the experiments 

 of de Vries, Correns, Hartley, East and Hayes, and himself, "all indicate 

 that certain somatic variations are inherited in strictly Mendelian fashion. 

 All these somatic variations consist in the appearance of self-colors on 

 plants that are normally variegated in pattern. The fact that variegated 

 plants occasionally throw both bud sports and seed sports with self- 

 colors is not, in general, to be taken as an indication that the variegated 

 plants in question are heterozygous. Such behavior seems to be insepa- 



