THE VOLATILE PART OF PLANTS. 57 
Wood, or 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 intergrown with a substance, (or sub- 
stances,) called dignin.* 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°|, 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 bouquets, are commonly made by dis- 
solving away the softer portions of fresh succulent plants by a hot solu: 
* According to F. Schulze, lignin impregnates, (not simply incrusts,) the 
cell-wall, it is soluble in hot alkaline solutions, and is readily oxidized by nitric 
acid. Schulze ascribes to it the composition 
Carbon iwgjveseeaausiesiicnsnionsperee 55.3 
Hydrogen... - 58 
OXYZel...cceeee dae odes wee wenden 38.9 
100.0 
This is, however, simply the inferred composition of what is left after the 
cellulose, etc., have been removed. Lignin cannot be separated in the puro 
state, and has never been analyzed. What is thus designated is probably a mix- 
ture of several distinct substances. 
Lignin appears to be indigestible by herbivorous animals, (Grouven, V. Hof- 
meister.) 
3 
