VARYING FACTORS 297 



kangaroo rat comes out at night and burrows deep in the sand 

 during the day. One of the ground squirrels avoids the hot sun 

 by running from one bush to another to get shade. Both of these 

 animals die if exposed to the sun for any length of time. 



Practical Exercise 8. Make a list of all the plants in your locality that are 

 dependent upon a large supply of water ; those which can exist with a very 

 small amount of water. 



Soil conditions. Plants grow only in soils to which they are 

 adapted. Some plants, as the blueberry, require acid soils, while 

 others are killed by acid in the soil. The liming of soils is one 

 example of how the farmer keeps the soil in condition for the crops 

 he is growing. The type of soil also affects the animals living in 

 them. Mud, sand, or clay will each contain different species and 

 numbers of plants or animals. Earthworms, for example, are not 

 found living in acid soils. 



Food conditions. For both plants and animals food is a factor 

 which determines the presence or absence of life. Mineral matter 

 is so necessary for the growth of plants that manure or artificial 

 fertilizer is employed to fill the need where the element nitrogen 

 is lacking. The presence of animal life in water is often dependent 

 upon the presence of plankton or minute forms of plant and 

 animal life which live near the surface of larger bodies of water. 

 Chemical substances necessary for plants and animals often de- 

 termine where they will live. 



Varying factors. Other factors, such as strong winds, electricity 

 in the atmosphere, the pressure of the air at different altitudes, and 

 the presence of dust or chemical fumes in the atmosphere, may all 

 play decided parts in determining what living things may exist in 

 given localities. 



Practical Exercise 9. List all the factors of the environment that affect 

 your daily life and give an example of how each one affects you. How does 

 water affect the life in your community? What effect has temperature on 

 plants in your locality ? Do you know any places near your home where un- 

 favorable factors in the environment prevent life? Are such factors forces 

 or things? Explain. 



Practical Exercise 10. Make a table naming all the factors of the environ- 

 ment which affect plants and animals and show how each factor affects both 

 plants and animals. 



Practical Exercise 11. From reference books, obtain a list of plants and 

 animals adapted to live under conditions lacking water ; on alkali plains ; in 



