26 THE VITAL FUNCTIONS. 



well in plants whose structure is wholly cellular ; a fact which 

 proves the 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 irreconcilable with the supposition that it is confined 

 in these vessels. 



Numerous experiments have been made to discover the ve- 

 locity with which the sap rises in plants, and the force it ex- 

 erts 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 

 fluid in the tube rose to a height, which, taking into account 

 the specific gravity of the fluid, was equivalent to a perpen- 

 dicular column of water of more than forty-three feet ; and, 

 consequently, exerted a force of propulsion considerably great- 

 er than the pressure of an additional atmosphere. The velo- 

 city, as well as the force of ascent, must, however, be liable 

 to great variation; being much influenced by evaporation, 

 and other changes, which the sap undergoes 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 movements of the cellular 

 structure already mentioned, the question is still enveloped in 

 considerable obscurity. There is certainly no evidence to 

 prove that it has any analogy to a muscular power ; and the 

 simplest supposition we can make is that these actions take 

 place by means of a contractile property belonging to the ve- 

 getable tissue, and exerted, under certain circumstances, and 

 in conformity to certain laws, which we have not yet succeed- 

 ed in determining. 



