Tissue Mixtures 271 



meristem in 68 plants. The first cell layer (L I), the second (L II), and 

 the third (L III, including everything below the first two) were often 

 affected independently. L I was changed most often and L II least. These 

 layers retained their specific chromosome complement throughout the 

 structure of the plant that developed from this meristem. These plants 

 are periclinal chimeras, not obtained from graft unions but by chemical 

 induction. Unlike the tomato-nightshade forms and similar ones, here 

 there may be three genetically different layers (2n, An, and 8n) instead 

 of two, and these three were found to occur in almost any order from 

 without inward. In the first report there were the following distributions 

 of polyploidy among the layers: 2n, 2n, An; 2n, An, 2n; 2n, An, An; 2n, 8n, 

 An; An, 2n, 2n; An, 2n, An; An, 8n, An; 8n, 2n, 2n; 8n, An, An. Other combi- 

 nations were found later. 



There is little difficulty here in distinguishing the layers since cell 

 size is approximately proportional to chromosome number and the 2n, 

 An, and 8n cells are thus markedly different. Since plants belonging to 

 the polyploid series differ little except in size, morphological combinations 

 of characters, as in the tomato-nightshade chimeras and others where the 

 partners are so unlike, cannot be seen. 



An understanding of periclinal chimeras has aided in the solution of 

 a number of horticultural and morphological problems. Not only have 

 the classical examples of the "graft hybrids" been given a satisfactory 

 interpretation but other facts discovered in vegetatively propagated 

 plants are now explained. Bateson (1921), for example, found that in a 

 variety of Bouvardia with pinkish-white flowers root cuttings produced 

 plants with red flowers. Here it is probable that the core of the plant 

 was of a red-flowered variety and only the outer layers were genetically 

 pink. Since lateral roots arise from the vascular cylinder ( here genetically 

 red) and push out through the cortex and epidermis, buds from these 

 lateral roots would be expected to form red flowers. Asseyeva (1927) ob- 

 served in the varieties of potatoes arising by bud mutation that if the 

 buds are removed from the seed tuber the new ones which now arise 

 from the deeper tissues form plants like those from which the mutant 

 variety had come. Zimmerman ( 1951a ) reports a similar case in roses. 

 Such vegetatively propagated plants are probably periclinal chimeras 

 with only the outer layer or layers of cells belonging to the mutant type. 



Other horticultural plants prove on examination to be chimeras. Einset 

 and his colleagues at the Geneva (N.Y. ) Experiment Station have 

 found that six large-fruited sports of several apple varieties are really 

 periclinal chimeras. The core tissues of the meristem are tetraploid, and 

 these are covered by one, two, or rarely three layers of diploid cells 

 (Einset, Blaser, and Imhofe, 1947; Blaser and Einset, 1948). 



Dermen has worked extensively with polyploid chimeras. In apples he 



