PROCEEDINGS OF THE FARMERS* CLUB. 367 



THE CAUSE OF CIRCULATION WITHIN A TREE. 



It has been ascertained, by introducing' coloring' matter into the trunk of 

 a tree, that the movement of the sap is upward through the wood alone; 

 •w^en the coloring matter has reached the nerves of the leaves its course is 

 changed, and it is found to be gradually moving down through the bark 

 nntil it reaches the bottom. Tiie cause of the action was at first supposed 

 to be the force of capillary attraction, but this does not account for the 

 fact that the sap will flow out of a vine when the top has been cut off. De 

 Saussure supplied the defect in this explanation by supposing that after 

 the action was commenced by capillary attraction, it was continued by the 

 alternate contraction and expansion of the tree, which, if true, does not 

 account for the continuous flow. The sap was supposed to return by its 

 own weight, but in drooping branches the return is in an upward direction. 

 According to Thouars the rise of sap is due to the expanding buds and the 

 development of leaves, which appropriate the juices in the vicinity; to sup- 

 ply the vacuum thus made the sap is forced up by atmospheric pressure; 

 but this view is erroneous, because the sap flows before the buds are formed. 

 Dutrochet, who first opened the path which Graham has successfully pur- 

 sued, ascribes the flow of sap to that force of diff'usibility which causes 

 two liquids separated by a porous sheet of animal membrane to intermin- 

 gle. He gave the name of Endosmose to the inward impulsion, and Exos- 

 mose to the outward impulsicm. By osmetic force water will pass through 

 a septum of animal membrane, covering the lower end of a glass tube in 

 •which there is a solution of sugar, and the action will continue until the 

 fluid has reached the top of the tube, when it will continue to flow out. 

 Vierodt states that the velocity of the current increases with the initial 

 concentration of the solution, but in a lower ratio. The numerous experi- 

 ments of Graham, recently made with mineral and animal septa, lead to 

 the inference that osmose is caused by the chemical action of the liquid on 

 •the septum. In attributing the circulation of sap in trees and plants to 

 cliemical action rather than to capillarity, we may have advanced in the 

 right direction, but it must be confessed that the subject is still enveloped 

 in mystery. 



According to Liebig, in annuals no new wood is formed, after the 

 month of August. All the carbon which the plant appropriates, after that 

 time, from the carbonic acid absorbed by the leaves, is employed in the 

 formation of nutritive compounds for the following year. Instead of woody 

 fiber, starch is formed and diffused through every part of the plant by the 

 autumnal sap. The chemical combination of carbon with the elements of 

 water, by a process which cannot, as yet, be imitated by man, forms woody 

 •fiber, gum, starch and sugar. M. Heyer states that starch deposited in the 

 body of a tree, can be recognized by its known form with the aid of a mi- 

 croscope. When by cavly cold weather, this provision is not made for the 

 tree, its growth during the next year will be checked. 



Why d(jes the saccharine liquid flow from the Maple more freely after a 

 frost ? This peculiarity is not observed in the Birch and other trees which 

 secrete the sweet principle. It would be accounted for on the Osmose 

 hypothesis, bj' supposing that the flow was checked by the frost, and 

 during that time the sap, already in the tree, became denser; this differ- 



