578 



Hoiv Living Things Affect One Another unit vii 



Fig. 330 A balanced aqnariiriii is a tiny zvorld of living things. What kinds of ani- 

 mals do you see? What siibstances do they get from tl.te plants? ]]lMit substances do 

 the plants get froi/i the animals? (schneider) 



the nitroijen-fixino" bacteria live inde- 

 ed Zj 



pendently in the soil; others live in nod- 

 ules (little growths) on the roots of 

 certain plants, such as bean plants, pea 

 plants, clover, alfalfa, and other mem- 

 bers of the legume family. This is why 

 planting clover or alfalfa or soybeans in 

 a field helps to improve the field and pro- 

 duce a better crop of some other plant 

 the next year. See Figures 328 and 329. 



Nitrogen compounds are also added to 

 the soil in another way — by the electric 

 discharge called lightning. During thun- 

 derstorms quite a large quantity of nitro- 

 gen in the air is combined with oxygen. 

 The compound is washed by rain into 

 the soil where nitrates are formed. 



By lightning and by the action of the 

 nitrogen-fixing and nitrifying bacteria 

 the supply of nitrates in the soil is kept 

 rather constant under natiu-al conditions. 

 Now do Exercise 6. 



The balanced aquarium. You can imi- 

 tate in the laboratory, in a limited wa\'. 



the conditions that occur on the whole 

 earth, and thus you can review briefly 

 what you have read. An aquarium is used 

 for this purpose. In order that the aquar- 

 ium ma\' imitate conditions on the earth, 

 which lies in space, it is necessary to iso- 

 late the aquarium, that is, to seal it. If 

 now you set up an aquarium with green 

 water plants, a few fish, and a few snails 

 and seal it, you will find that the plants 

 remain alive and so do the fish and the 

 snails. You know the reasons for this. 

 The animals and plants produce carbon 

 dioxide by oxidizing food; as a result 

 the green plants will have enough car- 

 bon dioxide for photosynthesis in which 

 both sugar and oxygen arc produced. 

 The fish and snails will then have enough 

 oxygen for their oxidation. If the proper 

 fish are chosen they and the snails will 

 eat the plants, thus making sure that the 

 plants will not overcrowd the aquarium. 

 In this \\'ay, too, the fish and snails get 

 food. The fish and snails in the meantime 



