i 3 2 SCIENCE PROGRESS 



But we arc left with the difficulty of explaining the mechanism 

 by which this occurs. Prof. Winkler was at first inclined to 

 believe that his hybrids were Hyperchimceras, plants in which 

 the tissues of the two parents were mingled together in a very 

 intimate manner, instead of being isolated in two distinct 

 longitudinal strips. This view he gave up, after a discussion 

 in the German Botanical Society had brought out the difficulties 

 which stood in its way. He then expressed the opinion that 

 they had arisen by a cell-fusion at the point of grafting — a view 

 vigorously combated by Strassburger. To Prof. Erwin Baur 

 belongs the credit of having made the suggestion which subse- 

 quent investigation has proved to be the correct solution of the 

 problem. 



He was engaged in the study of the heredity of the Pelar- 

 goniums, and he found, on examining anatomically the leaves of 

 those forms with white margins, that the organ consisted of a 

 core of green tissue surrounded by two or more layers of cells, 

 in which the chloroplasts were degenerate and contained no 

 chlorophyll — a hand of green tissue in a glove of colourless. At 

 the margin of the leaf the number of layers of cells is reduced till 

 finally only those with colourless chloroplasts persist, and thus is 

 produced the white margin. The idea of applying this arrange- 

 ment to explain the properties of the Solatium hybrids was 

 shortly afterwards made public. Thereupon Prof. Winkler 

 examined his plants to test the truth of the hypothesis. As it 

 happens, this is fairly easily done. Solanum lycopersicum possesses 

 24 chromosomes, S nigrum 72 : it was only necessary to count 

 the number of chromosomes in the different layers of the 

 vegetative points of the intermediate types. 



Prof. Baur's theory proved to be correct. The hybrids are 

 indeed plants in which a core of one parent is enclosed in a 

 skin of the other. Tilbingense consists of nightshade covered 

 by a single layer of tomato ; proteus has two layers of tomato ; 

 in Kcelreuterianum and Gwrtnerianum the tomato is the core 

 covered by one and two layers of nightshade respectively. We 

 are dealing, then, with plants which are not hybrids in the exact 

 sense of the word at all; they are, in fact, chimceras just as much 

 as the original object to which that term was applied, with this 

 difference — that, as the tissues are laid one over the other, and 

 not side by side, the result is a plant which exhibits characters 

 intermediate between those of the parents : to express this 



