Tissue Mixtures 261 



has long been known, for example, that successful grafts are usually 

 limited to closely related plants, either between those of the same 

 species or between species close together taxonomically. In exceptional 

 cases grafts can be made between genera and very rarelv between 

 families. Simon (1930), for example, has grafted Solarium and Iresine, 

 belonging to the widely separated families Solanaceae and Amarantha- 

 ceae. Nickell (1948) grafted white sweet clover on sunflower, and the 

 plants continued to grow with normal vigor for 5 months. Silberschmidt 

 (1935) studied 550 grafts between plants of the same species, of related 

 species, and of unrelated species. Anatomical fusion in some cases oc- 

 curred in grafts of unrelated species but here union was slower and less 

 extensive, passage of nitrogen from scion to stock was reduced, and 

 proteolytic activity of stock juices increased. The last fact is presumably 

 concerned with the failure of unrelated grafts. 



Schroter (1955) reports that Zinnia elegans, one of the Compositae, 

 can be grafted to tobacco, although not on some plants in the same 

 family as tobacco. He attributes the successful Zinnia grafts to the pres- 

 ence of nicotine in this plant. Mothes and Romeike ( 1955 ) grafted scions 

 of tomato, petunia, belladonna, and tobacco on tobacco stocks of vary- 

 ing nicotine content and found that the richer the stock was in nicotine, 

 the poorer was the development of the grafted scion. 



The passage of nicotine from tobacco roots to tomato scions has been 

 described (p. 220). Hieke (1942) found that in grafts between Lyco- 

 persicon, Nicotiana, Atropa, and Datura the alkaloids found in the scion 

 were those characteristic of the root to which this was grafted. 



Kostoff (1929) presented evidence from grafts between various plant 

 types that immunity, as tested by the precipitin reaction, can be acquired 

 by plants much as in animals. Chester and his colleagues, however, in a 

 series of papers (1932 and others) showed that the precipitates re- 

 ported are not the result of a true precipitin reaction but are simply 

 calcium oxalate, a widespread substance in plants. 



Monocotyledonous plants have been found more difficult to graft than 

 dicotyledonous ones, presumably because they lack a cambium. Muzik 

 and La Rue (1954), however, grafted a number of species of grasses, 

 including some belonging to different genera. 



The closer to the embryonic condition a tissue is, the more readily it 

 can be grafted. The smallest successful graft of this sort seems to be one 

 made by Gulline and Walker (1957) in which a shoot tip containing 

 only about 600 cells and with a volume of less than %ooo cu - mm - was 

 grafted back on the apex from which it had been cut. Later development 

 was normal. 



As to the reciprocal relations of stock and scion in nonwoody plants, 

 many conflicting results have been reported. Daniel described various 



