88 ON CAMBIUM. 



its natural sustenance ; the consequence of which is, 

 that the new layers, both of alburnum and of liber, which 

 should be annually produced by the descent of the cam- 

 bium, are not formed. 



The following season, therefore, the sap, instead of 

 rising through the soft and tender vessels of the newly- 

 formed alburnum must ascend through the alburnum of 

 the preceding year, under the additional disadvantage of 

 its being unusually hardened by the superabundant quan- 

 tity of nourishment it has received. 



This artificial mode of rendering alburnum hard and 

 mature, suggested the idea of stripping timber-trees of 

 their bark a year or two previous to their being cut down, 

 in order to harden the young external layers of wood, by 

 forcing the whole of the cambium to find a passage 

 through them, and thus convert the alburnum into per- 

 fect wood before the natural period. The experiment, 

 when first made, appeared to answer the most sanguine 

 expectations. The cambium, instead of forming new 

 layers of tender wood under shelter of the bark, forced 

 its way through the alburnum, giving it in one season the 

 hardness and consistence of perfect wood. But it was 

 afterwards discovered that the wood thus artificially ma- 

 tured, by being stripped of its bark, and exposed naked 

 and defenceless to the inclemency of the weather, to the 

 encroachment of lichens and creeping plants, and to the 

 attacks of insects and reptiles ; receives injuries, which 

 more than counterbalance the advantages of a preco- 

 cious maturity ; and render it totally unfit for building. 



Let us now turn, our attention to the composition of 

 the cambium, which subsequently becomes a component 

 part of the plant. 



We have observed, that about two thirds of the water 

 absorbed by the roots is evaporated by the leaves, one 

 third only remaining in the plant. This latter portion 

 exists in vegetables in two states : in the one it retains 



478. What is the consequence in the following season! 479. 

 What method of prematurely hardening timber trees has sometimes 

 been practised 1 ? 480. What was at first supposed to be the result'? 

 481. What disadvantage was afterwards discovered! 482. la 

 how many different states does the sap exist which remains in the 

 plant? 



