382 GENETICS IN RELATION TO AGRICULTURE 



to a single variegated specimen of Abutilon striatum Dicks., which was 

 introduced into Europe in 1868 and named Abutilon thomsoni. Miss 

 Reid has shown, however, that among the flowering maples (Abutilon) 

 the variegated forms can be grouped into two classes: "those with a 

 mottled variegation which is infectious and those with a non-infectibus 

 variegation with the white cells at the periphery. Both types are of 

 importance in horticulture, especially for use as bedding plants; both 

 types are of special scientific interest." 



The Physiological Behavior of Graft-hybrids. Although chromosome 

 counts and progeny tests indicate that the cells of each graft-symbiont 

 maintain their identity independently of the close proximity of foreign 

 cells, yet the intermediate characters of graft-hybrids indicate that the 

 components have a mutual influence upon each other. This influence 

 is especially notable in the manifestation of physiological activity in- 

 volving the whole plant. None of the Solanum graft-hybrids are as 

 vigorous as either component under normal conditions. In fact, it is 

 with considerable difficulty that they are maintained by means of cut- 

 tings, except in the case of Koelreuterianum. This lack of vegetative 

 vigor may not be characteristic of all graft-hybrids but it seems to be 

 common to most of them. Lack of vigor exists in many natural chimeras 

 also, especially in those involving chlorophyll reduction. 



In the Solanum graft-hybrids the germ cells of the two components 

 are not equally susceptible to the effect of adjacent foreign cells. In both 

 tubigense and Gaertnerianum the fruits contain fertile seeds which pro- 

 duce only pure nightshade plants. But in proteus only part of the seeds 

 are viable and these produce tomato seedlings, while in Koelreuterianum 

 the flowers are entirely sterile. Similarly, in Cytisus Adami the "hybrid " 

 (intermediate) flowers are sterile. 



Graft-hybrids offer many interesting possibilities as a means of studying 

 the physiology of development as influenced by the reciprocal relations 

 between the components. This method of attack has been utilized much 

 more extensively in the study of development in animals than in plants. 

 Crampton, for example, grafted together the pupae of different species 

 of moths thus producing double monsters. From the specific effects 

 upon pigmentation in some of the graft symbionts it was concluded that 

 the pigmental colors in some species are derived from the haemolymph 

 by processes of drying and decomposition which are regulated by some 

 specific internal factor. Crampton mentions several other investigators 

 who performed similar experiments on animals. Buder has suggested the re- 

 ciprocal grafting of male and female plants in dioecious species as a means of 

 investigating the physiology of sex determination in plants. This suggests 

 the whole field of reciprocal effects between scion and stock, concerning 

 which there has been considerable investigation in recent decades. 



