136 CIRCULATION OF THE SAP. 



the tree for succeeding years ; since the proper amount of al- 

 burnum is not deposited in the trunk, and from the hardening 

 of the previous alburnum, the sap for the succeeding year is 

 obstructed in its course. 



182. The cause of the ascent of the sap has been attribu- 

 ted to the evaporation of the leaves, to capillary attraction, 

 aided by the motion of the stem produced by the wind, to 

 endosmose, and to vital action. We believe it is generally not 

 due to any one of these, but to all of them, and we believe 

 more is due to vital action than to any other cause. That it 

 was wholly owing to vital action, in the first experiment 

 quoted from Hales, is evident from the facts, that none of the 

 ether alledged causes could act. Evaporation from the leaves 

 could not have produced it, for he states that there were no 

 branches on the stem subjected to experiment. Evaporation 

 or Endosmose cannot produce a Ibrce exterior to the body in 

 which they act. To vital action alone, then, we must ascribe 

 the principal force with which the sap is propelled. It seems 

 remarkable to us, that so much pains should be taken to ex- 

 plain phenomena on mechanical principles, which a.e wholly 

 impotent, when applied to the circumstances under considera- 

 tion. We are gravely told, and I quote high authority, that, 

 '* when a young bud is first excited to growth in the spring, 

 the fluids it contains are increased in density by evaporation ; 

 endosmose immediately takes place between it and the tissue 

 below it, which latter parts with the thinnest portion of its 

 contents, and then acts by endosmose upon the tissue below, 

 and thus the whole cord of vegetation is set in vibration. It 

 may be su[>posed that the mere effect of gravitation will carry 

 downwards the sap, in its densest state, after it has ceased to 

 obey the attraction of the leaves, and that it svill descend by 

 simple filtration till it reaches the roots ; but how we are to 

 account for its lateral transmission, through the medullary 

 rays, is still unknown". 



183. The first phenomenon quoted, is that by evaporation, 

 the fluids in the leaves are made more dense, which puts in 

 action endosmose. Now we are acquainted with no experi- 

 ment on the action of endosmose, where it ever separates the 

 fluids under its influence. We have no particular objection 

 to resorting to this ne.v agent in putting the sap in motion, 

 but we should like to know how this dense fluid, in the cell in- 

 to which the lighter fluid is entering by this power, is to be 

 discharged from the cell ? We have been unable, either from 

 our own experiments, or those recorded by others, to devise 

 any method. Endosmose, or Exosmose will not do it, for if 



