Chemical Basis of Genus and Species 47 



specificity was not yet fully recognized. The role of 

 lymphocytes in these cases has never been investigated. 

 The grafted piece always retained the characteristics 

 of the species from which it was taken. 



Plants possess no leucocytes and we therefore see 

 that they tolerate a graft of foreign tissues better than 

 is the case in animals. As a matter of fact heteroplastic 

 grafting is a common practice in horticulture, although 

 even here it is known that indiscriminate heteroplastic 

 grafting is not feasible and that therefore the specificity 

 is not without influence. The host is supposed to fur- 

 nish only nutritive sap to the graft and in this respect 

 does not behave very differently from an artificial nutri- 

 tive solution for the raising of a plant. The law of 

 specificity, however, remains true also for the grafted 

 tissues : neither in animals nor in plants does the graft 

 lose its specificity, and it never assumes the specific 

 characters of the host, or vice versa. The apparent 

 exceptions which Winkler believed he had found in the 

 case of grafts of nightshade on tomatoes turned out to 

 be a further proof of the law of specificity. Winkler, 

 after the graft had taken, cut through the place of 

 grafting, after which operation a callus formation oc- 

 curred on the wound. In most cases either a pure 

 nightshade or a pure tomato grew out from this callus. 

 In some cases he obtained shoots from the place 

 where graft and host had united, which on one side 

 were tomato, on the other side nightshade. What 



