Chap. 5 



ANIMALS AND THEIR ENVIRONMENTS 



79 



SR6LASER 



Fig. 5.13. Stems and leaves of pond lilies are nurseries for hatching eggs and 

 young animals, mainly invertebrates. A, strings of jelly that shelter minute eggs 

 of midges. B, eggs: on the under side of a lily leaf: 1 , snail; 2, water mite; 3, caddis 

 fly; 4, whirligig beetle; 5, beetle (Donacia); 6, beetle, the waterpenny (Psephenus). 



convert them into nitrates (NO3) — that are taken up by green plants, and 

 finally converted into the amino acids and proteins of green plants. Blue-green 

 algae are now known to fix nitrogen and the process may be even more 

 general than this. Many commercial fertilizers contain nitrates. 



Nitrogen-fixing bacteria are able to fix free atmospheric nitrogen in nitrog- 

 enous compounds which can be used by green plants. Some of these bacteria 

 live in the soil, estimated at least two billion to a teaspoonful in garden soil; 

 others live in nodules on the roots of clover, peas, and beans. The value of 

 these plants in building up the nitrogen supply in the soil is recognized by 

 farmers who rotate crops of clover with corn in order to supply the soil with 

 nitrogen which corn exhausts. Denitrifying bacteria occur in some soils. These 

 reverse the nitrifying process and reduce nitrites to free nitrogen which is then 

 released into the atmosphere. This is the nitrogen that is compounded with 

 water and brought to the earth in an electrical storm. The bolts of lightning 

 fix the nitrogen as nitrites and nitrates that are brought to the earth by the rain. 



Mineral Cycles. These include the time in which iron, phosphorus, or other 

 minerals are in the crust of the earth and in the body of a hving organism. 

 Calcium carbonate (CaCO:0 or lime is a good example for it is widely dis- 

 tributed in nature and an important component of bone. The developing 

 embryo of a mouse receives lime from its mother and after birth from its food, 

 notably milk. Lime is maintained in the body of the mouse, chiefly in its bones, 

 as long as it lives. Exactly the same storage of lime occurs in an elephant ex- 



