124 CIRCULATION OF THE SAP. 



upon the tissue below, and thus the whole cord of vegetation is 

 set in vibration. It may be supposed that the mere effect of 

 gravitation will carry downward the sap, in its densest state, 

 after it has ceased to obey the attraction of the leaves, and that 

 it will descend by simple filtration till it reaches the roots ; but 

 how w-e are to account for its lateral transmission through the 

 medullary rays is still unknown." 



228. The first phenomenon quoted is, that by evaporation 

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

 tion endosmose, or capillary attraction. Now, we are acquainted 

 with no experiment on the action of these forces, where they 

 ever separate the fluids under their influence. We have no 

 particular objection to resorting to these new agents in putting 

 the sap in motion, but we should like to know how this dense 

 fluid, in the cell into which the lighter fluid is entering by this 

 power, is to be discharged from the cell ? We have been un- 

 able, either from our own experiments or those recorded by 

 others, to devise any method. Endosmose, or Exosmose, will 

 not do it, for if we resort to exosmose, it can only pass out 

 into the ascending current, and by becoming lighter by dilu- 

 tion, is drawn by Endosmose immediately back again. But 

 our author solves the difficulty, by saying that gravity will 

 carry the denser sap downward ! True, but how comes the 

 denser sap separated from the lighter? and why does it not 

 return in the same vessels in which it ascends ? 



229. How does gravity operate in carrying the denser fluid 

 upward, as in many cases in which the extremities of branches 

 are lower than the point of insertion ? We know of no solution 

 to these questions, and we are compelled to say that they are 

 facts, which we can only refer to the action of that mysterious 

 principle which we call life. The action of this principle is, of 

 course, modified by. circumstances. It requires the action of ex- 

 ternal agents to call it into operation, and its force is increased 

 or retarded by the same. Heat and moisture exercise great in- 

 fluence over it in circulation. In the cold of winter it is nearly 

 suspended, but the warmth of spring calls it into action. After 

 its action has commenced with some vigor, a cold night seems 

 to retard or suspend its operations for the succeeding day. This 

 is seen in the Sugar-maple. The sap commences to flow from 

 the incisions, when the warm days and cold nights of spring 

 come on. But if several successive nights are so warm that it 

 does not freeze, the sap ceases to flow, and for the same reason 



228. What is the first phenomenon ? — 229. What difficulties in the way 

 of gravitation being the cause of the descent of sap? What is the cause? 

 How ilhistrated in the sugar-maple ? 



