Ch. III. 13] USES OF THE PLANT'S FOOD 99 



of the "n-alls occur, including the gelatixatiox familiar in 

 the Flax seed, while often the walls are also strongly infil- 

 trated ■s\ith mineral matters. 



The cell walls of a plant collectively form a continuous 

 system, somewhat like the cement walls and floors in our 

 modern buildings. In the compartments (the cells) lives 

 the protoplasm which builds the whole structure. Thus the 

 protoplasm, itself too soft and weak to rise from the grotmd, 

 can, like man, construct lofty buildings, in the rooms of which 

 it can dwell in the sun. 



It happens that the qualities which fit the cell walls for 

 their functions in plants make them also useful to man for 

 many of his needs. Hence he appropriates the elastic cel- 

 lulose for paper, or, a5 it occurs in long fibers, for cotton and 

 linen to make clothing. The waterproof cork serves to stop- 

 per his bottles. The stiff wood provides a rigid but easily- 

 worked material which he utilizes, as lumber, for his dwell- 

 ings, and as cabinet woods, for his furniture, while it serves 

 minor uses innumerable. 



JMan makes one other use of cellulose and its derivatives 

 not represented by any function in the plant, but dependent 

 on an incidental feature of their chemical composition, %-iz. 

 — they "ft-ill oxidize, or bum, thus pro^^ding him -nith fuel. 

 This use goes further than appears at first sight, for coal is 

 nothing but the cell walls of plants which throve in swamps 

 of the Carboniferous epoch, and in course of long ages, under 

 pressure and warmth, lost the two gaseous constituents, hy- 

 drogen and oxygen, retaining only the solid and oxidizable 

 carbon, which is the substance of coal. A perfect seciuence 

 can be traced from the photosjTithetic sugar made in the 

 green leaves of the Carboniferous plants, first to cellulose, 

 then in succession, with progressive loss of the gaseous con- 

 stituents, to hgnin, peat, soft coal, and anthracite. The 

 same qualities which make cellulose burn, make it explode, 

 in suitable combinations; and hence it is convertible into 

 high explosives, useful in peace and deadly in war. 



