120 



PLANTS AND MAN 



stems and leaves, have been found useful by man in numerous 

 ways. Since the days of primitive man, the wood fibers comprising 

 the trunks of woody Gymnosperms and Angiosperms have fur- 

 nished suitable material out of which to build shelters, household 

 articles, and conveyances for transportation on land and on 

 water. Wood and wood products can claim an importance 

 second only to plant foods. Other fibers, of a more flexible nature, 

 have long been used for weaving clothing materials and other 

 textile products; such pliable fibers are derived mainly from flax, 

 cotton, hemp, sisal and jute plants. Fibers from herbaceous 

 plants were used early in the history of civilization for making 

 paper, the best known being the Egyptian papyrus plant; in 

 more recent times the fibers from woody plants have become the 

 chief source. These aspects of man's 

 utilization of skeletal elements in the plant 

 body are more fully treated in Part Three. 

 The cellulose character of plants 

 makes them an excellent fuel, since cellu- 

 lose (like all carbon and hydrogen com- j 

 pounds) is very combustible. For heating | 

 and lighting his cave dwelling, early man 

 undoubtedly found wood the most avail- ? 

 able material. Even today, wood ranks at I 

 the top of the list of common fuels. If 

 woody stems, or the remains of non-woody 

 plants, chance to lie on the surface of the 

 ground, they are soon consumed by decay 

 bacteria and the oxidizable compounds of 

 the plant body are returned to the atmos- 

 phere as carbon dioxide and water. What- 

 ever potential energy such plants may have contained is thus lost 

 for man's use. However, if such plants happen to fall under 

 water, they escape the action of the decay bacteria. Mosses and 

 ferns, for example, often grow in or near swamps and ponds; 

 when they die, their remains settle to the bottom of the water 

 and form deposits of muck — in reality a water-soaked humus. 

 The cellulose materials in their tissues retain their potential 

 energy, since it is not released by the action of bacteria or fungi. 



Fig. 78. — Peat moss 

 accumulates in bogs 

 and can be used as fuel. 



