the Alburnum of Trees. 319 
elude that the sap ascends, and it is not, I think, difficult to con- 
ceive that this substance may give the impulse with which the 
sap is known to ascend in the spring. I have shewn that the 
bark more readily transmits the descending sap towards the 
roots than towards the points of the branches ;* and if the cel- 
lular substance of the alburnum expand and contract, and be so 
organised 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 shewn 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 opposite directions, 
and as well through inverted cuttings as others, is totally 
erroneous. •f 
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 then 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, in pro- 
portion to the vigour of the tree, and the state of the soil and 
season : and if the tubes which are thus filled be divided, the 
sap will flow out of them, and the tree will be said to bleed. 
But as soon as the leaves are unfolded, and begin to execute 
their office, the sap will be drawn from its reservoirs, and the 
tree will cease to bleed, if wounded. 
The alburnous tubes appear to answer another purpose in 
* Phil. Trans. 1804., p. 5. f Ibid. 
