152 ON THE ORIGIN AND OFFICE 



descending sap towards the roots than towards the points of the 

 branches * ; and if the cellular 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 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 opposite 

 directions, and as well through inverted cuttings as others, is totally 

 erroneous j-. 



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 trans- 

 piration. And the cellular substance, being then filled, may discharge a 

 part of its contents into the alburnous tubes, which again become reser- 

 voirs, and are filled to a greater or less height, in proportion 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 trees, and to 

 be analogous, in some degree, in their effects, to the cavities in the bones 

 of animals ; by which any degree of strength that is necessary, is given 

 with less expenditure of materials, or the incumbrance of unnecessary 

 weight ; and the wood of many different species of trees is thus made, at 

 the same time, very light, and very strong, the rigid vegetable fibres 

 being placed at greater distances from each other by the intervention of 

 alburnous tubes, and consequently acting with greater mechanical advan- 

 tage, than they would if placed immediately in contact with each other. 



I have shown in a former communication, that the specific gravity of 

 the sap increases during its ascent in the spring, and that saccharine 

 matter is generated, which did not previously exist in the alburnum, nor 

 in the sap, as it rose from the root : and I conceive it not to be impro- 

 bable, that the air contained in the alburnous tubes may be instrumental 

 in the generation of this saccharine matter. For I discovered in the 

 last autumn, that much air is absorbed, or at least disappears, during 

 the process of grinding apples for the purpose of making cider, and that 

 during this absorption of air, the juice of acid apples becomes very sweet. 

 See above, No. IV. t Ibid. 



