XVI INTRODUCTION. 



by the root of the stock is communicated slowly and 

 unwillingly to the scion ; under no circumstances is 

 the communication between the one and the other as 

 free and perfect as if their natures had been more nearly 

 the same ; the sap is impeded in its ascent, and the 

 proper juices are impeded in their descent, whence 

 arises that accumulation of secretion which is sure to 

 be attended by increased fertility. No other influence 

 than this can be exercised by the scion upon the stock. 

 Those who fancy that the contrary takes place ; that 

 the Quince, for instance, communicates some portion 

 of its austerity to the Pear, can scarcely have considered 

 the question physiologically, or they would have seen 

 that the whole of the food communicated from the 

 alburnum of the Quince to that of the Pear is in nearly 

 the same state as when it entered the roots of the for- 

 mer. Whatever elaboration it undergoes must neces- 

 sarily take place in the foliage of the Pear ; where, far 

 from the influence of the Quince, secretions natural 

 to the variety go on with no more interruption than if 

 the Quince formed no part of the system of the indi- 

 vidual. 



If we consider upon what principle the flavour of 

 particular fruits may be improved, we shall find that it 

 is entirely due to the increased action of the vital func- 

 tions of leaves. When the sap is first communicated 

 by the stem to the leaves, it has experienced but few 

 chemical changes since it first entered the roots. Such 

 changes as it has undergone have been due rather to 

 the solution of some of the pre-existing peculiar secre- 

 tions of the individual by the sap in its way upwards 

 through the alburnum, than to any other cause. As 

 soon, however, as it enters the leaves, it becomes al- 

 tered in a variety of ways, by the combined action of 

 air, and light, and evaporation ; for which purposes the 

 leaf is admirably adapted by its anatomical structure. 



