- On the Origin mid Office of the Alburnum, of Trees. 305 



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 I e 

 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 de- 

 gree of strength, that is necessary, is given with less expen- 

 diture 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 

 .Jrom each other by the intervention of alburnous tubes, and 

 consequently acting with greater mechanical advantage, 

 than they would if placed immediately in contact with each 

 other. 



I have shown in a former communication, that the spe- 

 cific gravity of the sap increases during its ascent in the 

 spring, ana 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 improbable, 

 that the air contained in the alburnous tubes may be inslrur 

 mental in the generation of this saccharine matter. For I dis- 

 covered in the iast autumn, that much air is absorbed, or at 

 Jeast disappears, during the process of grinding apples for 

 the purpose -of making cider, and that during this absorp- 

 tion of air, the juice of acid apples becomes very sweet, and 

 acquires many degrees of increased specific gravity ; and a 

 similar absorption of .air, with corresponding effects, is well 

 known to lake place in the process of malting. 



I shall conclude with observing, that in retracting the 

 opinion I formerly cnteriained respecting the asceot of the 

 sap in the alburnous tubes, 1 do not mean to retract any 

 opinion that I have given in former communications respect- 

 ing the subsequent motion of the sap through the central 

 vessels, the leaves, and bark ; or the subsequent junction of 

 the descending with the ascending current in the alburnum: 



Vol. 32. No. lt?7- Dec. 1S0S.. U cverv 



