PHYSIOLOGY OF VEGETABLES. 115 



nelian Cherry perspires, in the course of 24 hours, sev- 

 eral times its own weight. In moist weather, however, 

 plants sometimes absorb moisture by their leaves. 



The rapid waste of moisture, by the perspiration of 

 the leaves, and its supply by the absorption of the roots, 

 are both indicated by the rapidity with which a plant 

 wilts when plucked from its root, and its immediate 

 revival when the stem is pl&ed in water. It is indi- 

 cated also by the very great quantity of water ne- 

 cessary to supply some plants when placed in the house 

 in an espalier or flower-pot. 



4. Descent of the Sap and Growth of the Plant. 



It is very contradictory to the common notions of 

 vegetation, that plants should grow by a flow of sap 

 from the leaves downward ; yet this point seems to be 

 well established by Mi\ Knight's experiments. 



The sap, haying ascended from the roots to the 

 leaves, and being there modified and prepared, as has 

 just been described, descends through the vessels of 

 the bark, for the double purpose of nourishing and 

 increasing the plant, and of producing the various 

 secretions, such as gums, resins, turpentines, oils, &c. 

 which are usually found in the bark. " In the bark, 

 principally, if I mistake not, these peculiar secretions 

 of a plant are perfected, each, undoubtedly, in an ap- 

 propriate set of vessels." Smith. 



The new layers of bark and wood appear to be add- 

 ed in trees in the following manner. 



The descending cortical vessels of the liber, in the 

 growing season, throw out between the bark and the 

 wood, a pulpy or gelatinous matter, which has been 

 called cambium. The cambium, as it gradually becomes 

 fibrous and of a firmer consistence, separates into a 

 new layer of bark, which attaches itself to the other 

 layers, and, in its turn, secretes another cambium; and 

 a new layer of wood, which is deposited upon the outside 

 of the former layers. 



