PROBLEM I. 



Fig. 326 A covn}wn gray -green lichen, which 

 grows flat on a rock. This lichen is a partner- 

 ship between a fungus and an alga. What does 

 each contribute to the other? (hugh spencer) 



Why Plants and Animals Continue to Exist -^y^ 



money circulated from one person to the 

 next. We keep our silver dollars in con- 

 stant circulation. If someone kept them 

 tied up we would soon run short of sil- 

 ver dollars. The same thing is true of the 

 compounds used by plants. These com- 

 pounds kept in constant circulation con- 

 tain carbon, hydrogen, oxygen, nitrogen, 

 and the other elements found in proto- 

 plasm. Let us see how these elements are 

 kept in circulation. 



How oxygen and hydrogen are kept in 

 circulation. Whether they are green 

 plants, nongreen plants, or animals, all 

 living things use oxygen in respiration. 

 In spite of this, oxygen does not disap- 

 pear from the earth because oxygen is 

 released when green plants produce sugar 

 from carbon dioxide and water. The 

 same oxygen is used over and over again. 



Hydrogen is one element in water. In 

 photosynthesis water is used and the 

 hydrogen becomes tied up in sugar. This 

 sugar, as you will recall, is used in mak- 

 ing other sugars, starches, fats, and pro- 

 teins. These compounds may be oxidized 

 in the plant or may be eaten by animals 

 and oxidized there. In either case they are 

 changed into carbon dioxide and water. 

 Thus hydrogen is used over and over. 

 Exercise 4 will help you in reviewing 

 these ideas. 



How carbon is kept in circulation. You 

 know that green plants use carbon di- 

 oxide in the making of sugars. From 

 these they make starches, fats, proteins, 

 cellulose, and other carbon compounds. 

 If the plant is eaten by an animal, some 

 of these compounds become part of the 

 animal's body by assimilation, although 

 most of the food which the animal eats is 

 oxidized. Oxidation in both the plant and 



to determine \\ hether a relationship is one 

 of parasitism or symbiosis. If materials 

 are available you will enjoy Exercise 3. 



Food chains continue through the ages. 

 Whether or not animals or plants live in 

 special relationships of symbiosis or par- 

 asitism, they form a link in some food 

 chain, A particular food chain in a given 

 area may be broken but other relation- 

 ships establish themselves; life has con- 

 tinued throughout the ages and still goes 

 on. All food chains start with the manu- 

 facture of food in green plants. But can 

 plants continue to exist? It would seem 

 that they could because the compounds 

 used by plants in food manufacture are 

 probably as plentiful now as they were 

 millions of years ago. We have every 

 reason to believe that they will continue 

 to be as plentiful. 



Now the way to make a limited sup- 

 ply of anything last indefinitely is to 

 keep it in constant circulation. By using 

 it over and over again you can make a 

 very little go a long way. This is true of 



