122 Outline of Genetics 



were sectoral chimaeras, the tissues of one side of the branch being 

 those of the tomato, while the tissues of the other side of the branch 

 were night-shade tissues. Such sectoral chimaeras would not 

 infrequently later produce branches that were periclinal chimaeras, 

 having the tissues of one species inclosed within an envelope of the 

 other. That these were really periclinal chimaeras was established 

 by chromosome counts (tomato and nightshade having different 

 chromosome numbers), and by the fact that seedlings produced 

 by them were always of the species of the subepidermal tissue 

 from which the gametes arise. The periclinal chimaeras in turn 

 were observed at times to produce branches wholly of one or the 

 other of the parent-species, a performance which may well be 

 regarded as a type of somatic segregation. 



Fundamentally, the same behavior has been observed in 

 certain "natural" periclinal chimaeras (notably in types of Pelar- 

 gonium, Baur 2), involving white (deficient in chlorophyll) and 

 green tissues. The manner of origin of these natural chimaeras 

 is unknown, but it is quite possible that they arose as somatic 

 mutations. 



A very interesting case has been reported by Bateson (3) in 

 Bouvardia, which presumably may be something of the same sort 

 as the foregoing. Varieties of Bouvardia that are maintained true 

 to type by propagations from stem cuttings produce plants with 

 very different flower form, size, and color when propagated by root 

 cuttings. Since in normally produced buds of the stem both the 

 epidermis and the deeper lying tissues are maintained through 

 direct cell lineage, while the roots produced by stem cuttings arise 

 from the plerome and break through the periblem and dermatogen, 

 forming these parts anew, sprouts that develop from the roots 

 must have the genotype of the stele rather than that of the cortex 

 or epidermis. 



It is clear that classes i and 2 represent distinct types of 

 somatic segregation, the first arising as the result of irregular chro- 

 mosome distribution and the second from a segregation on the 

 part of tissues as a whole. Both might well be regarded as anom- 

 alies, since they are to be explained by irregularities in the com- 

 mon plant program. There remains to be considered one more type 

 of somatic segregation, and here, although no such finely balanced 



