114 THE SAP. [LECT. in. 



and have secretions deposited in its cells similar to 

 those of the parent, but to those of the stock. We 

 know, however, that this is not the case ; for, if the 

 branch of a Pear-tree be engrafted on an Apple-tree, 

 the Pear branch will produce leaves, flowers, and 

 fruit, and have the new wood formed on it exactly 

 the same as the Pear-tree from which it was cut, 

 although the first modifications of the nutriment 

 imbibed by the roots of the Apple stock are dif- 

 ferent from those of the parent Pear : but as soon 

 as it arrives at the secreting organs of the Pear 

 branch the alterations it undergoes are exactly the 

 same as if the branch had remained attached to its 

 parent stem ; and consequently the wood, leaves, 

 flowers and fruit, will have the characteristics of 

 its original. 



The modifications which take place in the roots 

 of plants, throw considerable obstacles in the way 

 of obtaining a perfect knowledge of this part of 

 the vegetable economy; for, to obtain such a 

 knowledge of the nature of sap would require an 

 examination of that fluid in a greater number of 

 different species of plants, than the opportunities, 

 and the period of any life, permit. All that we 

 can aim at, therefore, in the present state of 

 our knowledge, is the formation of a probable 

 hypothesis, rather than the attainment of truth 

 deduced from certain experiments. In this mode 

 of viewing the subject, we may regard the sap 



