36 THE VITAL FUNCTIONS. 



sap, precipitate^ 1 fr m it hy the separation of the greater part 

 of the water ill, ;ld it in solution. The cambium becomes, 

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

 the organizatu >roper to the plant of which it now forms 

 an integrant pm^: it constitutes two layers, the one, belong- 

 ing to the wood, being the alburnum; the other, belonging 

 to the bark, being the liber. 



The alburnum and the liber, which have been thus con- 

 structed, perform an important part in inducing ulterior 

 changes on the nutrient materials which the returning sap 

 continues to supply. Their 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 possessvery different sensible 

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

 proportions of thejr 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 simple cellular tissue.* 



In the 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 analyses of Dr. Prout, the following- is the composition 

 of these substances: 1000 parts of 



Pure Gum Arabic consist of 586 of oxygen and hydrogen, united in the 



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

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

 Pure crystallized Sugar, - - 572 - - - 428 - - - 

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



