104 THE SAP. [LECT. in. 



their action is most energetic in spring and at 

 midsummer, at which periods, therefore, a much 

 greater quantity of fluid is found in the vegetable 

 vessels. As, however, the simple examination of 

 the sap itself is our present object, it is sufficient 

 to state the fact ; and to know that, at these sea- 

 sons, when an incision is made through the bark 

 and part of the wood of most kinds of trees, or a 

 hole is bored in the trunk, a fluid exudes in con- 

 siderable quantity. This is the sap, or common 

 juice. It is in the same situation, for the purposes 

 of the plants, as the chyle of animals is, while it 

 is yet in the thoracic duct, and before it is mingled 

 with the blood, and exposed in the lungs to be 

 fitted for the purposes of the animal. Neither is 

 in a proper state for yielding the various secretions, 

 and adding, by the process of assimilation, to the 

 growth of the plant, or of the animal ; but the 

 analogy goes no farther. In the animal, the di- 

 gestive powers of the stomach and the action of 

 the mesenteric glands so change the food taken 

 into it, that no chemical analysis of the chyle 

 produced from it could lead to an accurate know- 

 ledge of the kind of food, which had been em- 

 ployed by the animal ; but, in plants, the food is 

 already prepared in the ground before it is ab- 

 sorbed by the roots, and, therefore, were it pos- 

 sible to obtain the sap from the vessels very near 

 to the extremities of the roots, we should be en- 



