The Plant and Its Environment 13 



He does not, therefore, try to improve all the environmental 

 factors that affect his plants, but gives his attention to those that 

 require it most. He irrigates if the soil lacks water. He adds 

 fertilizer if the soil is deficient in certain minerals. Insect injuries 

 he tries to prevent by spraying the plants with substances that 

 kill the insects. These are all efforts directed toward removing 

 or taking care of the limiting factors. It is often difficult to 

 determine, in a particular case, what the limiting factor may be. 

 But the ever increasing study of effects of environmental factors 

 on plants is every year making the discovery, and the correction, 

 of the limiting factors in crop production easier and more efficient. 



Distribution of plants determined by the environment. Every 

 plant has a somewhat definite form and structure. In order that 

 it may live, it must carry on certain chemical and physical 

 processes. Because of these processes and its particular structure 

 and composition, a plant also has certain indispensable require- 

 ments as to its environment. The requirements of some plants 

 are very definite, of others more or less indefinite ; and the re- 

 quirements of different plants vary greatly. If we survey any 

 extensive tract of land, we see that its surface is more or less 

 broken into elevations and depressions containing streams, lakes, 

 or ponds. Slopes extend in various directions and are thus 

 differently exposed to sunlight. The soil may be shallow on 

 some slopes, and deep on others ; it may vary in texture and 

 mineral components. Some areas may be wet or moist, and 

 others dry. In some places the soil is fertile, in others more or 

 less sterile. Each of these smaller divisions of the land affords 

 different opportunities and conditions for plant growth, and 

 under natural conditions we find very different plants growing 

 on them. 



Habitats. The smaller areas into which every large land sur- 

 face is broken are characterized by various groups and combina- 

 tions of environmental factors. These areas from the stand- 

 point of plant distribution are called habitats. Lakes, ponds, 



