50 



THE AMERICAN NATURALIST [Vol. XL V 



As yet no seed has been obtained from crosses between 

 the graft-hybrids themselves. 



The Nature of Graft-Hybrids 

 Winkler concluded at first that all the graft hybrids 

 except the chimeras probably arose from actual cell 

 fusion and might be compared directly with hybrids 

 arising from true fertilizations. It was suggested by 

 another student of graft-hybrids, Bauer. 4 that these ap- 

 parent true hybrids might also he chimarras of a type 

 which he has called ' ' periclinal, " i. e., the outer tissues 

 are derived from one parent, and the inner tissues from 

 the other, but none of the tissues themselves are of 

 hybrid nature. This hypothesis seemed the more prob- 

 able from the results of investigations of MacFarlane 

 upon Cytisus Adami 5 in which he showed that the epi- 

 dermal tissues were strikingly like those of C. purpureus 

 while the inner tissues were like those of C. laburnum. 

 An investigation of the Cratcego-mespilus hybrids re- 

 vealed a similar state of affairs. 



Acting on this suggestion Winkler made a careful cyto- 

 logical study of his hybrids and found that four of them 

 were indeed periclinal chimaeras. But one of them 

 seemed to be a real hybrid resulting apparently from a 

 fusion of cells at the junction of the graft and stock. 



The nuclei in the nightshade and the tomato differ 

 very much in the number of the chromosomes so that the 

 determination of the origin of the tissues in the hybrid 

 is made comparatively easy. The chromosome number 

 in the sporophytic tissue is twenty-four in the tomato 

 and seventy-two in the nightshade. These numbers 

 were found in the tissues of all of the graft hybrids ex- 

 cept 8. danriniamim where the reduced number of the 



4 Bauer, Erwin, "Fropfl.astar.lo," Biolofiim'hr* Coit mlblatt, 33, No. 15, 

 497-514, 1910. 



