CHAPTER 10 



Tissue Mixtures 



In animal bodies such combinations of genetically unlike tissues as grafts, 

 mosaics, and chimeras are rare in nature and rather difficult to bring 

 about experimentally, but in plants these are much commoner and easier 

 to produce. This is presumably due, in large measure, to the presence of 

 localized growing points in plants, which knit together readily. The arts 

 of grafting and budding have long been known to horticulturalists and 

 provide means for combining two or more varieties of related plants and 

 especially for vegetative multiplication of types that cannot be propa- 

 gated by seeds or in which cuttings do not easily root. 



In chimeras and in localized genetic changes, tissue mixtures may 

 be much more intimate than in ordinary grafting and provide oppor- 

 tunities for a study of organization and tissue relationships which are 

 not available in homogeneous plant bodies. 



STOCK-SCION INTERRELATIONS 



In the practice of grafting, a small branch or shoot, the scion, is in- 

 serted into a larger rooted portion, the stock, by means of a cleft or 

 other opening in such a way that meristematic regions of the two come 

 into contact. The same result is achieved in a somewhat simpler fashion 

 by budding. Here a bud from one type is slipped under a cut in the bark 

 of another so that the two cambia are in contact, the bud later growing 

 into a shoot. A third piece, in "double-worked" trees, is sometimes 

 inserted as an intermediate between stock and scion. In practice, these 

 methods are used chiefly in woody plants and are the means by which 

 most horticultural varieties of trees and shrubs are propagated. The prob- 

 lems in these plants will therefore be discussed first. 



A question of much importance both practically and theoretically 

 concerns the effects produced by the stock on the scion or the scion on 

 the stock. What substances can, and what cannot, pass from one to the 

 other across a graft union? Do the two graft partners remain in com- 

 plete physiological isolation save for the passage of water and solutes 



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