l4 SAP. 



sufficiency of this, there can scarcely be a doubt, for 

 we have already learned,* that when the life of a plant 

 is destroyed by electricity, though its texture is unin- 

 jured, it ceases to bleed, and its fluids remain stationa- 

 ry* But still they are exposed to all the other agents 

 which have been thought to accelerate their motion. 



The ascending sap is usually insipid and inodorous, 

 being according to the experiments of the French 

 chemists but slightly impregnated with those substan- 

 ces, by which plants are nourished, and from which 

 their secretions are formed. 



As soon as the leaves expand, this excess of moisture 

 is exhaled, chiefly through the pores of their epider- 

 mis, and the fluid thus perspired appears scarcely dif- 

 ferent from pure water, being the residue of sap de- 

 prived of the elements of those secretions which are 

 formed during vegetation ; and though it escapes unseen, 

 its amount is in some cases very considerable. A 

 ?mall plant of the Sunflower was found to exhale more 

 than a pound in the course of a day, and the Corneti 

 tree of Europe, perspires still more freely. 



But the sap does not ascend indiscriminately, through 

 every portion of the stem. A tree may be divested of 

 its bark, and it will produce its usual supply of leaves, 

 and an increased quantity of fruit. Mr. Knight remov- 

 ed the pith from a young shrub, and it continued to 

 jtow with its former luxuriance. The nutriment there- 

 fore does not ascend ' through the bark or pith. Nor 

 does it pass through the central portion of the woody 

 stem, for its vessels are nearly consolidated, and they 

 are not, even during the bleeding season, filled with a 

 limpid fluid. But one channel remains, and it is the 

 alburnum of trees and shrubs, or the analagous por- 

 tions of herbaceous plants, and hence we learn why the 

 * Page 25. 



