COMPOSITK^N OF LIVING TIIINCS 31 



and eggs of some animals. On the other hand, cells may he so minute that 

 in the ease of the plant cells named bacteria, several million could Ix! placed 

 on the dot of this letter i. The forms of cells may be extremely \ aried in 

 different tissues ; they may assume the form of cubes, columns, spheres, 

 flat plates, or may be extremely irregular in shape. One kind of tissue 

 cell, found in man, has a body so smaU as to be quite invisible to the naked 

 eye. although it has a prolongation several feet in length. Such are some 

 of the cells of the nervous sj'stem of man and other larse animals, as the 

 o.x, elephant, and whale. 



I Varying Sizes of Living Things. — Plant cells and animal cells may 

 iKe alone or they ma.v form collections of cells. S(jme plants are so simple 

 in structure as to be formed of only one kind of cells. Usually living 

 organisms are composed of se\-eral groups of different kinds of cells. It is 

 only necessary to call attention to the fact that such ccjllections of cells 

 may form organisms so tiny as to be barely visible to the eye ; as, for in-^ 

 stance, some water-lo\'ing, flowerless plants or many of the tiny animals 

 li^•ing in fresh water or salt water, such as the hydra, small worms, and tiny 

 crustaceans. On the other hand, among animals the bulk of the elephant 

 and whale, and among plants the big trees of California, stand out as no- 

 table examples. 



^I 



'Relation to Organic and Inorganic Matter. — The inorganic 

 matter covering the earth, as air and water, and forming the great 

 mass of its bulk, is made use of by plants and animals. The latter 

 make their homes in earth, air, or water ; thej^ take in the oxygen 

 of the atmosphere; they use the water for drinlcing; but in the 

 main their food eonsists of organic matter. Plants, on tl^other 

 hand, manufacture food out of the dead organic and li^ganic 

 matter contained in the soil, air, and water, and then change this 

 food into the living matter of their own bodies. This organic 

 matter in turn may become food for animals. 



In the last chapter we found that the classes of substances in 

 an animal or plant and the organic food substances have a similar 

 composition. Let us now consider chemically the substance which 

 forms the basis of all living things. 



i Chemical Composition of Protoplasm. — Living nititter, when 

 "'analyzed by chemists in the laboratory, seems to have a very Com- 

 dex chemical composition. It is somewhat like a protcid in that 

 it always contains the element nitrogen. It tilso contains the ele- 

 ments carbon, hydrogen, oxygen, and a little sulphur. Calcium, 

 iron, silicon, sodium, potassiimi, phosphorus, and other mineral 



