GRAFT-HYBRIDS AND OTHER CHIMERAS 377 



the germ cells arise from the sub-epidermal layer. The seedlings of 

 Gcertnerianum are also pure nightshade because, although this form 

 consists of tomato tissue within, it is enveloped by two layers of night- 

 shade cells. Similarly with proteus, which is a nightshade except for its 

 two outer layers of tomato cells, the fruits resemble tomatoes and from 

 the seed pure tomatoes have been raised. The other form, Kcelreuterianum, 

 fails to produce fruits. The chromosome numbers are 24 for the tomato 

 and 72 for the nightshade. If a fusion of nuclei involving diploid numbers 

 had occurred the cells of the supposed hybrids should contain 96 chromo- 

 somes, but the only counts obtained by Winkler in the four chimeras 

 were 24 and 72. Thus it appears that in each graft-symbiont the two 

 kinds of tissue maintain their identity. Yet there is a combined effect 

 on the morphological characters. The physiological interactions too 

 are such as to cause reduced vigor. This effect is least noticeable in 

 Kodreuterianum which is sterile. The fifth new form, which Winkler 



FIG. 153. Diagram to show formation of adventitious buds arising at the point of 

 union of the two graft-components A and B. a, represents a sectorial combination; b, a 

 periclinal combination. (After Buder.) 



claims is a true graft-hybrid, was named Darwinianum. It appeared on 

 one of the shoots from a decapitated graft. Winkler claims the chromo- 

 some number of this form is 48 and that certain if not all of the tissues of 

 this plant are composed of cells derived from the actual fusion of tomato 

 with nightshade cells which involved nuclear fusion. If this is actually 

 the case S. Darwinianum is a hybrid in the strict sense and the only one 

 known to have been produced by vegetative means. However, Baur 

 points out that Winkler bases his claim for this number on the ground 

 that he found 24 chromosomes in the pollen mother-cells which arise 

 from the sub-epidermal layer. Baur thinks that Winkler's interpretation 

 is unwarranted. He believes it much more probable that S. Darwin- 

 ianum is a periclinal chimera with a nightshade epidermis, then a sub- 

 epidermal layer of tomato cells and the adjoining inner tissues of night- 

 shade. Then the number found, 24, is the diploid number of the tomato 

 and the reduction division, according to this explanation, is omitted in 

 the tomato pollen mother-cells which, in this chimera, are bounded on 

 both sides by nightshade tissues; or else it occurs at an unusually late 

 stage in development. "I cannot admit," says Baur, "that the exist- 

 ence of real graft-hybrids in the strictest sense of the word is proven." 



