Chap. 4 PLANTS PROVIDE FOR THEMSELVES AND THE ANIMALS 53 



ture peculiar to plants, their strong cell walls composed of cellulose, or cellu- 

 lose impregnated with lignin if the tissue is woody. 



The Plant Cell Wall. Plant cell walls have long provided heat and power 

 for humanity (Fig. 4.2). Whether lignified or not, cellulose burns rapidly in 

 combination with oxygen; its stored energy is released in the form of heat and 

 it is converted back to carbon dioxide and water. When cellulose is subjected 

 to heat and pressure for long periods of time it undergoes chemical changes; 

 hydrogen and oxygen are removed and solid carbon remains. This is what 

 happened in the ancient swamps and forests where peat, lignite, and coal were 

 formed, one or another product depending upon the material and the stage of 

 the carbonization. Coal exposed longer and under the right conditions becomes 

 graphite; exposed still further and properly conditioned, it crystallizes as pure 

 coal, or with extreme hardness as diamonds. The heated live coal of the open 



Fig. 4.2. Typical plant cell. In plant cells the cytoplasm occupies a relatively 

 small space and the central part contains one or more large vacuoles filled with 

 watery solution containing many substances related to the life processes of the 

 plant. The vacuoles are separated from the protoplasm by an almost invisible semi- 

 permeable membrane (or tonoplast), a lively and important region of exchange of 

 substances. In contrast to animal cells those of plants have a prominent cell wall 

 strengthened by cellulose, made woody by lignin. (Courtesy, Rogers, Hubbell, and 

 Byers: Man and the Biological World, ed. 2. New York, McGraw-Hill Book Com- 

 pany, 1952.) 



