266 The Phenomena of Morphogenesis 



(1937) also found that Mailing stock had some influence on the form 

 and size of Mcintosh apples grown on it. These cases need confirma- 

 tion. 



In general, from the mass of literature available, one may conclude that 

 stock can influence scion in producing differences in plant size, size of 

 leaves and fruits, plant habit, flowering time, life span, content of inor- 

 ganic and certain organic substances and growth substances, and to some 

 extent in fertility and resistance to disease. Influence of scion on stock 

 is much less marked. Most of these effects definitely have a chemical or nu- 

 tritional basis, and few cases of strictly qualitative changes are known. 

 It should be recognized, however, that a purely quantitative difference, 

 as in fruit or leaf size, may influence shape by allometric correlation 

 (p. 105). Truly morphogenetic effects rarely-perhaps never-pass across 

 a graft union. The great preponderance of evidence also supports the con- 

 clusion that no permanent genetic change is induced by one graft part- 

 ner on the other. 



Stock-scion relationships have been reviewed by Rogers and Beak- 

 bane (1957). Much of the literature on grafting (and many other 

 things) is summarized in Krenke's (1933) monumental work. The theo- 

 retical aspects of grafting have been reviewed by Roberts ( 1949 ) . 



CHIMERAS 



The instances of tissue mixture just described have been by the arti- 

 ficial union of two genetically different plants. These types remain sharply 

 separated, each branch or other unit of the plant belonging definitely 

 to one or to the other. There are tissue mixtures, however, where the fusion 

 is much more intimate than this and where an organ such as a stem, leaf, 

 or root is not homogeneous but is made up of two or more tissues that are 

 genetically unlike. This difference may arise by somatic mutation, the 

 mutated cells multiplying and forming a part of the whole, or it may be 

 the result of a mixture of meristematic tissues at a graft union. The im- 

 portant fact, morphogenetically, is that these diverse groups of cells 

 do not each form an organism or produce developmental abnormalities 

 but that they coexist as parts of the same organized system. What is pro- 

 duced is a normal, whole plant. Here the organizing capacity of living 

 stuff and the self-regulatory quality of the organism are particularly 

 conspicuous. 



Mixtures of tissues that come from different sources are called chimeras, 

 a term proposed by Winkler (1907b) from the analogy between such 

 plants and the chimeras of mythology which were part lion, part goat, 

 and part dragon. A number of types of chimeras are recognized, depend- 

 ing on the relationship of their components. 



