304 On the Origin and Office of the Alhurmnn of Trees, 



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 dis- 

 charged from them exceeded more than a hundred times that 

 which the tubes could have contained at the time the inci- 

 sions were made, even had every tube been filled to the ex- 

 tremity of the moot distant branch. And, as it has been 

 shown that the sap can pass' up when all the alburnous tubes 

 are intersected, 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- 

 clude that the sap ascends, and it is not, I think, difficult to 

 conceive that this substance may give the impulse with which 

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

 that the bark more readily transmits the descending sap to- 

 wards the roots than towards the points of the branches*^ 

 and if the cellular substance of the alburnum expand and 

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

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

 other direction, it will be readily impelled to the extremities 

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

 often repeated in the writings of naturalists, of a power in 

 the alburnum to transmit the sap with equal facility in op- 

 posite directions, and as well through inverted cuttings as 

 others, is totally erroneous + . 



If the sap be raised in the manner I have suggested, much 

 of it will probably accumulate in the alburnum in the spring; 

 because the powers of vegetable life are, at that period, more 

 active than at any other season ; and the leaves are not then 

 prepared to throw off any part of it by transpiration. And 

 the cellular substance, being ihen filled, may discharge a 

 part of its contents into the alburnous tubes, which again 

 become reservoirs, and are filled to a greater or less height, 



* Philosophical Transactions for 1804, g.. 5. f Ibid. 



