ENVIRONMENT OF LIVING THINGS 23 



If we were able by careful analysis to reduce a plant and an animal 

 to the chemical compounds of which they were formed, we should discover 

 that both contained mineral material. We have just seen examples of 

 this in plants. Mineral matter is found in bone, in the shells covering 

 mollusks (clams, snails, etc.), and in other parts of the bodies of animals. 



Water in Living Things. Water forms an important part of the sub- 

 stance of plants and animals. This can easily be proved by weighing a 

 number of green leaves, placing them in a hot oven for a few moments, and 

 then reweighing. The same experiment made with a soft-bodied animal, 

 as the oyster, would show even more water than in leaves. Some jelly- 

 fish are composed of over 90 per cent water. The human body contains 

 about 65 per cent water. 



Gases Present. Some gases are found in a free state in the bodies of 

 plants or animals. Oxygen is of course present wherever oxidation is 

 taking place, as is carbon dioxide. Other gases may be present in minute 

 quantities. 



Problem III. The foods that living organisms need. (Lab- 

 oratory Manual, Prob. III.} 



Composition of Living Matter. The living part of a plant or 

 animal is made up of the elements carbon, hydrogen, oxygen, and 

 nitrogen, with a very minute amount of several other elements, 

 which collectively we may call mineral matter. The living part 

 of a plant corresponds closely in chemical composition to the living 

 part of an animal. The sugar found in grains or roots of plants 

 has nearly the same chemical formula as the animal sugar found 

 in the liver of man ; the oils of a nut or fruit are of composition 

 closely allied to the fat in the body of an animal. These build- 

 ing materials of a plant or animal may be placed in one of the 

 three following groups of substances : carbohydrates, materials 

 containing a certain proportion of carbon, hydrogen, and oxygen; 

 fats and oils, which contain chiefly hydrogen and carbon with less 

 oxygen; and nitrogenous or proteid substances, which contain 

 nitrogen in addition to the above-mentioned elements. The above 

 three kinds of organic materials also form a large part of the foods 

 of all animals and plants. 



Foods. What is a food f We know that if we eat a certain 

 amount of proper foods at regular times, we shall be able to go on 

 doing a certain amount of work, both manual and mental. We 

 know, too, that day by day, if our general health is good, we may 



