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 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 further elaboration in 

 its composition ; 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 ingredients ; and we may infer that the 

 real chemical alterations, which are required in 

 order to effect these conversions, are compara- 

 tively 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 



