262 The Phenomena of Morphogenesis 



instances of a marked qualitative effect of stock on scion or scion on 

 stock. The production of tubers, for example, he believed could be trans- 

 ferred by grafting from the Jerusalem artichoke to the common sun- 

 flower, which normally bears no tubers. In one of his last papers (1929) 

 Daniel maintains that these induced changes have sometimes become 

 transmissible through seed for several generations and regards this as 

 proof of the inheritance of an acquired character. 



This is essentially the position taken by Lysenko and his Russian 

 colleagues. One of them, Avakian (1941), reported marked reciprocal 

 effects, on fruit color and other characters, of red-fruited, yellow-fruited, 

 and white-fruited tomatoes when grafted together in various ways. Simi- 

 lar experiments were repeated in this country by Wilson and Withner 

 (1946), who were unable to confirm these results in any respect. Bohme 

 (1954) also found that no inheritable effects were produced by grafting 

 between varieties of tomatoes. 



There are a number of well-authenticated cases, however, of the trans- 

 mission, between graft partners, of factors that determine qualitative 

 and not simply quantitative and nutritional differences. These present 

 some important problems both for morphogenesis and for physiology. 

 Conspicuous among them are the numerous instances where a flowering 

 stimulus, from a plant which has been induced to flower by a particular 

 photoperiod, can be transferred by grafting to a nonflowering plant and 

 cause it to flower (p. 396). Evidently some substance is transmitted across 

 the graft that stimulates flowering. 



The effect of this stimulus may be modified in various ways. Haupt 

 (1954), using a late-bearing variety of peas, grafted terminal shoots of 

 different ages on stocks of different ages. If scions of young plants are 

 grafted on older ones, flowering takes place up to six nodes earlier than 

 in controls grafted to stocks of their own age. Evidently the substance con- 

 cerned with flower development is not formed in the first stages of 

 the plant's growth but can be effective then if introduced from older 

 plants. 



The production of flowers after grafting may be due to other factors 

 than a specific flower-inducing substance. The Jersey type of sweet po- 

 tato rarely flowers in this country but can be made to do so by grafting 

 it to another species of Ipomoea that does not form storage roots (Kehr, 

 Ting, and Miller, 1953). These authors believe that flowering results 

 from the accumulation of carbohydrates in the shoots after grafting. 



The tendency to form tubers in potato may be transmitted by grafting. 

 If the shoot of a variety that produces tubers under a long day is grafted 

 to a short-day variety and grown under long days, the short-day variety 

 will now produce earlier and larger tubers than it would have done by 

 itself (Howard, 1949). This effect was not transmitted through these 



