36 THE VITAL FUNCTIONS. 



sap, precipitated from it by the separation of the o-fcater part 

 of the water that held it in solution. Tlie cambium becomes, 

 in process of time, more and more consolidated, and acquires 

 the organization proju^' to the ])lant of which it now form.s 

 an inte2;rant part: it constitutes two layers, the one, belona;- 

 ins to the wood, l^einir tlie all)urnuni; tlie other, belonirinj^ 

 to the bark, beinii the liber. 



The all)urnum and the liber, which have been thus con- 

 structed, perform an important part in inducing ulterior 

 changes on tlie nutrient materials which the returning sap 

 continues to supj)ly. Tlieir cells absorb the gummy sub- 

 stance from the surrounding fluid, and by their vital powers 

 effect a still farther elaboration in its composition; convert- 

 ing it either into starch, or sugar, or lignin, according to the 

 mode in which its constituent elements are arranged. Al- 

 though these several principles possess very different sensible 

 properties, yet they are found to differ but very slightly in the 

 proportions of their ingredients; and we may infer that the 

 real chemical alterations, which are required in order to ef- 

 fect these conversions, are comparatively slight, and may 

 readily take place in the sim])le cellular tissue.* 



In tiie series of decompositions which are artificially ef- 

 fected in the laboratory of the chemist, it has been found 

 that gum and sugar are intermediate products, or states of 

 transition between various others; and they appear to be pe- 

 culiarly calculated, from their great solubility, for being easi- 

 ly conveyed from one organ to another. Starch and lignin, 

 on the other hand, are compounds of a more permanent 

 character, and especially adapted for being retained in the 

 organs. Starch, which, though solid, still possesses consi- 



• According to the aniilyscs of Dr. I'rout, the following- is the composition 

 of these siibstunces: 1000 parts of 



Pure Gum Arabic consist of 586 of oxygen and liydrogcn, united in tlie 



proportions in which they exist in water, and 414 of carbon. 

 Dried Starch, or Fecula, of 560 water, and 440 - - - 

 Pure ciystallized Sugar, - - 572 - - - 428 - - - 

 Lignin from Boxwood, - - - 500 - - - 500 - - - 



