Tissue Mixtures 267 



The work on chimeras has been reviewed by Swingle (1927), Weiss 

 ( 1930), Neilsoh-Jones ( 1934, 1937), and Cramer ( 1954). 



In the so-called mixed chimeras the two kinds of tissue are mingled 

 irregularly together. This mixture may persist but it is often a tem- 

 porary stage and succeeded by one of the more regular types as the 

 meristem becomes better organized. In mericlinal chimeras, often de- 

 rived from mixed ones, one type of tissue forms a thin layer over a part 

 of the surface of the other. 



The other types show a more regular relation between their two 

 components. In sectorial chimeras, a definite sector of a radially sym- 

 metrical structure such as a root, stem, or fruit is of one type and the 



Fig. 10-1. Sectorial chimera in apple. (From Zundel.) 



rest is of the other (Fig. 10-1). It is not uncommon to find in fruits 

 such as apple or orange a sector in which the color or texture of the 

 skin is different from that of the rest and which sometimes can be traced 

 into the axis of the fruit. Such a sector may be distinguished in the stem, 

 also, and the line between the two components sometimes runs out through 

 the blade of a leaf. The term sectorial chimera may be used more 

 broadly for a type in which there are large masses of diverse tissue 

 adjacent to each other, regardless of whether the boundary line has 

 any relation to the axis of symmetry. Thus an apple in which the terminal 

 portion is of one type and the basal another, with an irregular boundary 

 between, has been called a sectorial chimera. Sectorial chimeras are 

 often found in shoots that arise from the vicinity of a graft union. Some 

 of them may really be mericlinal ones, with one member covering a 



