Babcock and Lloyd: "Somatic Segregation" 



83 



ting might occasionally deviate from 

 the regular method by which each 

 chromosome is exactly halved, but 

 the observations of many investigators 

 indicate that mitosis in undifferentiated 

 tissues certainly proceeds with remark- 

 able precision and regularity. Never- 

 theless, the idea of "somatic segrega- 

 tion" has become fixed in many minds; 

 so much so, indeed, as to make it 

 apparently the most convenient term 

 with which to "explain" many varia- 

 tions, especially in plants. Even true 

 vegetative mutations resulting in chi- 

 meras and bud sports have been attrib- 

 uted to "somatic segregation," Bateson^ 

 having originally proposed this explana- 

 tion of the production of nectarines on 

 peach trees. This hypothesis assimies 

 that the smooth skin of the nectarine is 

 due to the loss of a unit character 

 possessed by the peach. But on this 

 basis, Bateson was unable to explain the 

 reported production of peaches by a 

 nectarine tree. How could a unit 

 character once lost be recovered again' 

 Clearly the only rational explanation of 

 these phenomena will be expressed in 

 terms of reversible chemical changes in 

 genetic factors. Based on this concep- 

 tion of germinal variations it appears 

 that somatic mutations, like germinal 

 mutations, are caused by alterations in 

 specific factors which occur without the 

 intervention of any known irregularity in 

 chromosome behavior. As examples from 

 the abundant evidence in favor of this 

 view, the researches of Morgan and his 

 associates^ on the pomace fly, Drosophila 

 ampelophila, have demonstrated that 

 germinal mutations are due to specific 

 factor changes, and Emerson'* in his 

 study of the inheritance of variegation 

 in maize has shown that bud mutations 

 also, must be due to alterations in spe- 

 cific factors. 



Hence, those who accept the chromo- 



some and factorial hypotheses of hered- 

 ity find it unnecessary to resort to the 

 conception of "somatic segregation" in 

 order to explain bud sports and chimeras. 

 In fact the employment of this expres- 

 sion seems to be actually misleading. 

 Besides, there is a tendency to resort 

 to this conception in attempting to 

 explain other variations in individual 

 plants which can be more reasonably 

 accounted for on some other basis. A 

 case in point is the occurrence of 

 deciduous and persistent calyx lobes 

 in pome fruits. 



AN ILLUSTRATIVE CASE 



Tufts ^ examined 5,496 fruits from 

 a single tree of Le Conte pear and 

 56 fruits from a tree of Transcendant 

 crab and noted whether the calyx lobes 

 were deciduous or persistent. In the 

 case of the pear it was found that 3,780 

 fruits had all the calyx lobes deciduous 

 while 1,616 had one or more lobes 

 persistent. Of the crab fruits, 32 had 

 all calyx lobes deciduous and 24 had 

 one or more lobes persistent. We shall 

 consider in detail only the data from 

 the pear. The 1,616 fruits were classi- 

 fied according to the number of persis- 

 tent lobes present as follows: 5 P {i. e., 

 with 5 persistent lobes) — 773; 4 P — 116; 



3 P— 200; 2 P— 289; 1 P— 238. 

 Without considering the significance 



of the ratio of 3,780 fruits having all 

 lobes deciduous to 1,616 fruits having 

 one or more persistent, Tufts reduced 

 his data on the basis that each of the 

 five carpels found in every normal pear, 

 with its corresponding calyx lobe, is an 

 "individual." Thus the 238 fruits hav- 

 ing one persistent lobe were taken as 

 5 X 238 or 1,190 individuals, 238 

 of which had persistent lobes and 



4 X 238 or 952 had deciduous lobes. 

 By this method he derived the fre- 

 quencies 21,235 deciduous to 5,745 



3 Morgan, T. H., Sturtevant, A. H., MuUer, H. J., and Bridges, C. B. The Mechanism of 

 Mendelian Heredity, 1915. 



■* Emerson, R. A. The Possible Origin of Mutations in Somatic Cells. Awer. Nat.47. 1913. 



The Inheritance of a Recurring Somatic Variation in Variegated Ears of Maize. Amer. 

 -Va/. 48 and Nebraska Agr. Exp. Sta. Research Bull. 4. 1914. 



Anomalous Endosperm Development in Maize and the Problem of Bud Sports. Zeits. f. 

 ind. Abstam. u. Vererb. XIV, 5. 1915. 



* Tufts. W. P. An Inquiry into the Nature of a Somatic Segregation of Characters in the 

 Le Conte Pear. Oregon Agr. Exp. Sta. Bull. 123. 1914. 



