26 THE VITAL FUNCTIONS. 



in all cases necessary for its conveyance. In 

 many instances the sap is known to deviate from 

 its usual rectilinear path, and to pursue a cir- 

 cuitous course, very different from that of any of 

 the known vessels of the plant. The diffusion 

 of the sap in different directions, and its sub- 

 sidence in the lowest parts, on certain occasions, 

 are facts irreconcileable with the supposition 

 that it is confined in these vessels. 



Numerous experiments have been made to 

 discover the velocity with which the sap rises in 

 plants, and the force it exerts in its ascent. 

 Those of Hales are well known : by lopping off 

 the top of a young vine, and applying to the 

 truncated extremity a glass tube, which closed 

 round it, he found that the fluid in the tube rose 

 to a height, which, taking into account the spe- 

 cific gravity of the fluid, was equivalent to a per- 

 pendicular column of water of more than forty- 

 three feet ; and consequently exerted a force of 

 propulsion considerably greater than the pressure 

 of an additional atmosphere. The velocity, as 

 w^ell as the force of ascent, must, however, be 

 liable to great variation ; being much influenced 

 by evaporation, and other changes, which the sap 

 imdergoes in the leaves. Various opinions have 

 been entertained as to the agency by which the 

 motion of the sap is effected ; but although it 

 seems likely to be resolved into the vital move- 

 ments of the cellular structure already mentioned, 



