ON THE ALBURNUM OF TREES, 31 



sence of the sap in the alburnous tubes, during that part of 

 the year in which trees, when wounded, bleed abundantly, 

 does not afford any decisive evidence of the ascent of the 

 sap through those tubes. 



In the last spring, when the buds of the sycamore first Experiments 

 began to prepare tor unfolding, I found, that the sap ^ e * he * T ° a * 

 abounded in the points at the annual branches; and at the 

 same time it flowed abundantly from incisions made into the 

 alburnum near the root. But when similar incisions were 

 made at the distance of eight or ten feet from the ground, 

 not the least moisture flowed; and the tubes of the albur- 

 num appeared to contain air only. I also observed, that the 

 sap flowed as abundantly from the upper as from the under 

 side of the lower incisions, if not more abundantly, and so 

 it continued to flow to the end of the bleeding season. 



The sap must therefore have been, by some means, thrown 

 into the tubes above the incisions, for the quantity discharged 

 from them exceeded more than a hundred times that which 

 the tubes could have contained at the time the incisions were 

 made, even hud every tube been filled to the extremity of 

 the most distant branch. And, as it has been shown, that 

 the sap can pass up when all the alburnous tubes are inter- 

 sected, there appears, I think, sufficient evidence, that it • 

 must in this case have been raised by some other agent than 

 those tubes. 



Through the cellular substance I therefore venture to con- The sap as- 



elude that t'oe sap ascends ; and it is not, 1 think, difficult to cends through 



conceive, that this substance may give the inipulse, with substance. 



which the sap is known to ascend in the spring. I have shown, 



that the bark more readily transmits the descending sap 



towards the roots than towards the points of the branches*; 



and if the celltflar substance of the alburnum expand and 



contract, and be so organized as to permit the sap to escape 



more readily upwards from one cell to another, than in any 



other direction, it will be readily expelled to the extremities 



of the branches : and I have shown, that the statement, so ™ 



r- j • i • • ,» ,. The sap not 



otten repeated in the writings of naturalists, of a power in transmitted 



tbe alburnum to transmit the sap with equal facility in op- Wlth e( l ual fa * 



9 r cihty in oppo- 



• Phil. Trans. 1804, p. 5 ; ©r Journal, vol. X, p. 292. 



posite 



