PROCESS OF DEVELOPEMENT. CHAP. IV. 



Attributed An opinion pretty generally entertained was that 

 sap. by which the induration of the alburnum, and its 

 consequent durability, are attributed to the loss of 

 sap which the layer sustains after the period of its 

 complete developement ; when the supply from the 

 root diminishes, and the waste by evaporation or 

 otherwise is still kept up, inducing a contraction 

 or condensation of its elementary principles that 

 augments the solidity of the layer, in the first 

 degree, and begins the process that future years 

 finish. 



Theory But Mr. Knight believes the induration of the 

 Knight, alburnum as distinguishable in the winter to be 

 owing rather to some substance deposited in it in 

 the course of the preceding summer, which he re- 

 gards as being the proper juice in a concrete or 

 inspissated state, but which is carried off again by 

 the sap as it ascends in the spring. This was ori- 

 ginally a conjecture which he thinks he has proved 

 to be also the truth. The first argument is founded 

 on the fact that the sap as it ascends becomes gra- 

 dually more and more mixed with the proper juice 

 of the plant, as may be seen in the case of the 

 Maple, and Birch, the sap of which last when ex- 

 tracted near the root, being almost without taste ; 

 but when extracted at the height of seven or eight 

 feet, or less, being sensibly sweet. The second ar- 

 gument is founded on the fact that the specific 

 gravity of the sap in the bleeding season increases 

 according to the height at which it is extracted. 



