778 ECOLOGY 



in large part retain their individuality and are nearly as distinct from 

 one another as are a parasite and its host. The scion may well be com- 

 pared to such a water parasite as the mistletoe, since it derives water and 

 inorganic salts from the host or stock, while it manufactures its own 

 carbohydrates and proteins. The chief difference between a scion and 

 a parasite is that the former is organically united to the stock, while the 

 latter has haustorial processes which ramify through the host tissues; 

 however, there is no sharp delimitation between the phenomena, of 

 parasitism and those of grafting, since scions sometimes develop roots 

 in the stock. 



The influence of the stock and the scion upon each other. Many cases 

 are now known in which either the stock or the scion is influenced by 

 the other, so that a part of the original individuality is lost; particuLirly 

 in evidence is the influence of the stock upon the scion. 1 Such differ- 

 ences may manifest themselves in physiological behavior, as in ch; nged 

 respiration and synthesis, and particularly in reproductive phenonena; 

 or there may be changes in form, in color, or in chemical composition, as 

 when an apple scion grafted on the wild crab bears more acid fruit. 

 The best known changes concern the time of fruiting; a variety of the 

 apple that requires ten or fifteen years to come into bearing from seed 

 may bear in a year or two if a twig from a sapling is grafted on an old 

 stock, while a twig from an old stock grafted on a sapling does not fruit 

 for years. Some late apples ripen earlier when grafted on a stock of 

 an early variety. Certain species of Citrus are more productive when 

 grafted on Citrus trifoliata than when growing independently. When 

 the morning glory, which is an annual, is grafted on the sweet potato, 

 which is a perennial, the latter develops its characteristic tuberous roots 

 much earlier than otherwise, thus giving an excellent illustration of the 

 influence of the scion upon the stock. When the sunflower, which is 

 an annual, is used as a stock for the Jerusalem artichoke, whic i is a 

 tuberous perennial, the artichoke scion develops aerial tubers ard the 

 sunflower stock is characterized by a large development of secondary 

 wood. 



Investigators differ as to whether a chemical compound that is char- 

 acteristic of a given species or variety can pass into a stock or scion of 



1 In spite of the great array of evidence against it, some able investigators still adhere 

 to the theory that stocks and scions maintain the individuality of their respective species; 

 the remarkable changes here recorded are said to represent only such differences as 

 might be obtained by growing the plants in different soils and climates. 



