22 THE VITAL FUNCTIONS. 



Several opinions have been entertained with re- 

 gard to the channels through which the sap is 

 conveyed in its ascent along the stem, and in its 

 passage to its ultimate destination. Many obser- 

 vations tend to show, that, in ordinary circum- 

 stances, it is not transmitted through any of the 

 distinguishable vessels of the plant : for most of 

 these, in their natural state, are found to contain 

 only air. The sap must, therefore, either traverse 

 the cells themselves, or pass along the intercellular 

 spaces. That the latter is the course it takes is the 

 opinion of De Candolle, who adduces a variety of 

 arguments in its support. The sap, he observes, 

 is found to rise equally well in plants whose struc- 

 ture is wholly cellular ; a fact which proves that 

 vessels are not in all cases necessary for its con- 

 veyance. In many instances the sap is known to 

 deviate from its usual rectilinear path, and to pursue 

 a circuitous course, very different from that of any 

 of the known vessels of the plant. The diffusion of 

 the sap in different directions, and its subsidence in 

 the lowest parts, on certain occasions, are facts 

 irreconcileable with the supposition that it is con- 

 fined in these vessels. 



Numerous experiments have been made to dis- 

 cover 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 ex- 

 tremity a glass tube, which closed round it, he found 

 that the f^uid in the tube rose to a height, which, 

 taking into account the specific gravity of the 

 fiuid, was equivalent to a perpendicular column of 

 water of more than forty-three feet ; and conse- 



