WATER REQUIREMENTS FOR PLANT GROWTH 191 



light, soil and many other factors. These factors are variable 

 and in a state of nature work in combination. Therefore, the 

 presence of any particular species of plant in any locality is 

 dependent on all the environmental factors: which influence its 

 growth. The study of the distribution of plants over the sur- 

 face of the earth is known as plant geography. It will be seen 

 that plant geography depends on plant ecology, the latter being 

 a study of the environmental factors influencing plant growth 

 and the former a study of the distribution of plants as a result 

 of the influence of environmental factors. 



Crop Zones. — We frequently hear references to the crop 

 zones or belts of the United States. Some of the most prominent 

 are the cotton belt, the corn belt, the winter wheat belt, the spring 

 wheat belt, the potato-growing districts, the riceH-growing dis- 

 tricts, etc. These areas are dependent entirely on the environ- 

 mental factors just referred to. But there are many smaller 

 areas which are just as well or better defined than the large areas. 

 There are also the great arid and semi-arid districts of the far 

 West and Southwest, much of which can never be irrigated. The 

 ,'igricultural development of a country depends largely on a 

 knowledge of the factors involved in plant growth. A knowl- 

 edge of these factors is also necessary in order that valuaHe 

 plants can be introduced from other parts of the world and 

 grown advantageously. Some crops will grow under a variety 

 of conditions and vsdll frequently vary in character in accord- 

 ance with the conditions. The hard wheats are grown under 

 semi-arid conditions, while the soft wheats are grown under con- 

 ditions with a higher water supply. 



LABORATORY EXERCISES. 



The grouping of plants into societies can be observed in any locality. 

 This chapter should be followed by a study of plants in the field. 



