268 The Phenomena of Morphogenesis 



sector but only skin deep. Sectorial chimeras may sometimes be dis- 

 covered by a study of their internal structure (Brumfield, 1943, p. 76, 

 and Fig. 10-2). 



Of most interest to morphogenesis, however, are periclinal chimeras. 

 In these remarkable plants the outer cell layers are derived from one 

 graft partner and the entire inner portion of the plant from the 

 other. "Graft hybrids," which have long been a puzzle to horticultural- 

 ists, prove to be periclinal chimeras. They arise from grafted plants and 

 partake of certain of the characters of each, but their own characters 

 cannot be transmitted through seed. One of the familiar forms is 

 Crataegomespihis, originating from a graft between two rosaceous 

 genera: Crataegus, the hawthorn, and Mespilus, the medlar. Another is 



Fig. 10-2. Sectors of a root of Vicia after previous exposure to X radiation. Stippled 

 cells are those in which observed mitoses showed that the chromosomes were un- 

 changed. Black cells are those where a chromosomal change could be observed. 

 These cells presumably are descended from a cell at the very tip in which a change 

 had been induced and which had then given rise to a sector or wedge of similar cells. 

 ( From Brumfield. ) 



Cytisus-labiirnum, coming from a graft between these two leguminous 

 genera. 



Plants of this sort were first experimentally produced and carefully 

 studied by Winkler ( 1907fc, and later papers ) . He grafted two closely 

 related species, the nightshade, Solarium nigrum, and the tomato, Solanum 

 hjcopersicon. After union, most of the scion was cut off, and from 

 adventitious buds arising near the point of union plants developed 

 which sometimes showed mixtures of the two types of tissue. Most of 

 these were sectorial chimeras. Occasionally, however, Winkler found 

 a plant that showed no obvious separation into two types of tissue but 

 was clearly intermediate in character between nightshade and tomato. 

 Several distinguishably different types of such "graft hybrids" appeared in 

 these experiments, were maintained by vegetative propagation, and bore 



