THE VOLATILE PAET OF PLANTS. 57 



Wood, 01- woody fiber, consists of long and slender cells 

 of various forms and dimensions, see p. 271,) which are deli- 

 cate when young, (in the sap wood,) but as they become 

 older fill up interiorly by the deposition of repeated layers 

 of cellulose, which is intergrovvn with a substance, (or sub- 

 stances,) called lignin* The hard shells of nuts and 

 stone fruits contain a basis of cellulose, which is impreg- 

 nated with ligneous matter. 



When quite pure, cellulose is a white, often silky or 

 spongy, and translucent body, its appearance varying some- 

 what according to the source whence it is obtained. In 

 the air-dry state, it usually contains about 10° I,, of hygro- 

 scopic water. It has, in common with animal membranes, 

 the character of swelling up when immersed in water, from 

 imbibing this liquid ; on drying again, it shrinks in bulk. 

 It is tough and elastic. 



Cellulose differs remarkably from the other bodies of 

 this group, in the fact of its slight solubility in dilute acids 

 and alkalies. It is likewise insoluble in water, alcohol, 

 ether, the oils, and in most ordinary solvents. It is hence 

 prepared in a state of purity by acting upon vegetable 

 matters containing it with successive solvents, until all 

 other matters are removed. 



The "skeletonized" leaves, fruit vessels, etc., which compose those 

 beautiful objects called phantom houqueU, are comujoiily made by dis- 

 solving away the softer portions of fresh succulent plants by a hot solu- 



* According to F. Schnlze, lignin impregnates, (not simply incmsts,) the 

 cell-wall, it is soluble in hot allcaline solutions, and is readily oxidized by nitric 

 acid. Schulze ascribes to it the composition 



Carbon 55.3 



Hydrogen 5.8 



Oxygen 3S.9 



100.0 

 This ie, however, simply the inferred composition of what is left after the 

 cellulose, etc., have been removed. Lignin cannot be separated in the pure 

 state, and has never been analyzed. What is thus designated is probably a mix- 

 ture of several distinct substances. 



Lignin appears to be indigeBtible by herbivorous animals, {Orouvm, V. Hof- 

 meister.) 



3* 



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