RETURN OF THE SAP. 41 



and acquires the organization proper to the plant 

 of which it now forms an integrant part : it con- 

 stitutes two layers, the one, belonging to the 

 wood, being the alburnum ; the other, belonging 

 to the bark, being the liber. 



The alburnum and the liber, which have been 

 thus constructed, perform an important part in in- 

 ducing ulterior changes on the nutrient materials 

 which the returning sap continues to supply. 

 Their cells absorb the gummy substance from 

 the surrounding fluid, and by their vital powers 

 effect a still further elaboration in its compo- 

 sition ; converting it either into starch, or sugar, 

 or lignin, according to the mode in which its 

 constituent elements are arranged. Although 

 these several principles possess very different 

 sensible properties, yet they are found to differ 

 but very slightly in the proportions of their in- 

 gredients ; and we may infer that the real che- 

 mical alterations, which are required in order to 

 effect these conversions, are comparatively slight, 

 and may readily take place in the simple cellular 

 tissue.* 



In the series of decompositions which are arti- 



* 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 carbon. 



Pure crystallized Sugar . . 572 428 



Lignin from Boxwood . . . 500 500 



