The Various Substances Made by Plants 113 



Chemically, cellulose is related to grape sugar and formed there- 

 from in much the same way that starch is, its formula being the 

 same as that of starch, (C 6 H 10 5 )n, with the n, however, represent- 

 ing a different but unknown value. 



Although cell-walls when young consist only of cellulose, in 

 some structures they become penetrated later by other materials 

 which are probably formed by alteration of 

 the cellulose itself, and which give new proper- 

 ties to the walls. Thus, it is a stiffening sub- 

 stance called lignin, added to cellulose walls, 

 which converts them into wood, and also 

 forms other hard tissues, such as the shells of 

 nuts; while a very different substance, cutin 

 or suberin, makes the walls thoroughly water- 

 proof, as they are in all cork, and in the thin 

 waterproof epidermis which ensheaths the en- 



FIG. 36. A cell with 



tire plant. An alteration of the cellulose of parts of four others, 



,, , . i j ,, ., from the interior of 



another kind produces the mucilaginous ma- the nut of ivory Palm, 

 terial displayed when some seeds (e. g., those tnSj 

 of the Flax), are placed in water, or when deposition of layers 



of cellulose, through 



fallen leaves turn gummy on sidewalks in wet, which run canals P er- 



.1 i i i i-i mitting a continuity of 



warm, autumn weather; and such also is the protoplasm from one 

 origin of the mucilage or slime found in des- cel1 cavityto auother - 

 ert plants on the one hand and water plants on the other, with 

 peculiar functions in those plants to be later considered. 



There are highly consequential facts of another kind about 

 cellulose whether lignified or not. It burns readily in presence 

 of oxygen, being converted back in the process to carbon dioxide 

 and water, the very substances from which it was originally made. 

 When, however, it is subjected for a long period of time to pres- 

 sure and heat, gradually it undergoes definite chemical changes 

 through which its hydrogen and oxygen are removed, leaving 

 behind the solid and non- volatile carbon. This is exactly what 

 has happened in the case of the plants which grew of old time in 



