332 Plant Physiology 



In the case of some other fruits, greater hardiness or 

 resistance to cold is secured by grafting upon hardy stocks. 

 The sweet orange is now commonly grafted upon the 

 rough lemon and upon the sour orange, both in Florida 

 and California, although some believe that the quality of 

 the orange is thereby somewhat affected. Several species 

 of American grapes are notably resistant to Phylloxera 

 (especially Vitis rotundifolia and V. riparia), and these 

 vines are now commonly employed as stocks in certain 

 sections of southern Europe where this insect has done 

 great damage. Since the insect is mainly injurious upon 

 the root, there is a direct advantage in using American 

 stocks. 



The transmission of certain diseases, or pathological 

 conditions, such as peach yellows, contagious chlorosis, 

 etc., may occur by grafting, but in general it is felt that 

 there is relatively little of what may be termed special 

 chemical influence of the stock upon the scion. Litera- 

 ture is full, however, of contradictions and strife regarding 

 the mutual influence of stock and scion. An hereditary 

 effect has been claimed, but the lack of definite work with 

 strains sufficiently pure, renders the whole matter proble- 

 matical. 



196. Forcing. This term is rather loosely employed. 

 It may signify merely the production of plants out of 

 season, generally under glass or other protection, such as 

 the growing of tomatoes in the winter; again, it may 

 suggest the growing of plants which, in a particular lati- 

 tude, require certain well-controlled conditions. These 

 applications of the term require no further consideration 

 physiologically. When, however, it is implied that fore- 



