THE INTERDEPENDENCE OK LIVING THINGS 



:><> 



The other mmeral components of living matter, of wliich sulpliur 

 phosphorus, calcium, potassium, and iron are among tlie most impor- 

 tant, are all found either in water, soil, or both. How the plant makes 

 use of them and turns them over for the use of animals is an interesting 

 story to be told later. But enough has been said to show that foods 

 made by the green plants form the supply on which all animals live. 



carbohydrates^,, .^^^^ carbon iiox.ae^^ /'0>^yg«?'v 



proteins^ X -^-ureot /^ >\ n'til«ts 



slltsZI^ AnimaU (Gr^enPlantC r^f"' 



>.^ter- \^ y-sctlts V y^-^^'■-1^<.^ 



The food relationships between green plants and animals. 



Life Habits of Bacteria 



In this web we call life, bacteria play a most important part. Since 

 bacteria contain no chlorophyll, they are unable to make carbo- 

 hydrate food, and must obtain their foods from decaying organic 

 matter. In order to absorb such food it must be made soluble so 

 that it will pass into their bodies. This they do by digesting food 

 substances by means of enzymes ^ which they secrete. Bacteria 

 that grow or thrive in the presence of oxygen are called aerobic, while 

 those which live without free oxygen are called anaerobic. The latter 

 need oxygen, like other living things, obtaining it by breaking down 

 the foods on which they live, and utilizing oxygen freed in this process. 



Relation of Bacteria to Free Nitrogen 



It has been known since the time of the Romans that the growth of 

 clover, peas, beans, and other legumes causes soil to become more 

 favorable for the growth of other plants, but the reason for this was 

 not discovered until modern times. On the roots of the plants 

 mentioned are found little nodules, or tubercles, in each of which 

 are millions of nitrogen-fixing bacteria {Rhizobium leguminosarum) , 

 that take nitrogen gas from the air between the soil particles and 

 build it into nitrites which arc tlu>n converted by otluM- bacteria 

 (Nitrobacter) into nitrates. In this form it can be used by plants. 

 Nitrogen-fixing bacteria live in a symbiotic relationship with the 

 plants on which they form tubercles, their hosts pro\iding them with 

 organic food. 



J See pages 127-128. 



