Tissue Mixtures 



273 



ment (Fig. 10-4). This is particularly true in the polyploid chimeras. 

 The work on Datura by Satina and Blakeslee has provided much infor- 

 mation here, and Dermen (1953) presented similar evidence for the 

 peach. There seems to be no invariable rule as to just what mature 

 structures are produced by different layers at the meristem. The particu- 

 lar tissues contributed by L II and L III are not only different among 

 species but between large and small individuals of the same species 

 (Dermen, 1951). 



It is significant that in the root, where a root cap is present and con- 

 tinuous layers of cells do not cover the tip of the meristem, periclinal 

 chimeras do not occur. Sectorial chimeras, however, have been ob- 

 served in roots (p. 76). 



Fig. 10-5. Somatic mutation. Sectors of cells in corolla of Pharbitis resulting from 

 mutation from colorless to colored sap. The wider the sector, the earlier was the origin 

 of the mutation in the development of the flower. ( From Imai and Tobuchi. ) 



SOMATIC MUTATIONS 



Mixtures of various types of tissue may appear not as a result of graft- 

 ing or experimental treatment but spontaneously. Mutations in vegetative 

 cells are not uncommon in some plants. Where a mutant cell is dis- 

 tinguishable, by color or in other ways, its descendants form a spot or 

 stripe of tissue unlike the rest (Fig. 10-5). The earlier the mutation oc- 

 curs, the larger the mass of tissue that will be produced. In annual 

 Delphinium, Demerec (1931) found a gene that mutated frequently in 

 petal cells, changing their color from rose to purple. An early mutation 

 altered a large part of the plant but later ones formed only small spots 

 on the petals. Some cases of variegation, as in maize pericarp (Anderson 

 and Brink, 1952), are due to mutable genes of this sort though most 

 color patterns in plants result from differentiation during development 

 and not from a mixture of genetically different tissues. Some genetically 

 variegated plants become chimeras, and Dermen ( 1947b ) has been able 

 to determine the specific meristematic layer (LI, L II, or L III ) in which 

 the mutation took place. 



