PLANT ASSOCIATIONS 273 



we have come to recognize that certain plants are naturally 

 associated, because they grow in the same conditions. Any 

 set of conditions for plants is said to be a habitat, that is, a 

 place that certain plants inhabit. Therefore, each kind of 

 habitat has its own association of plants, and a plant associa- 

 tion may be denned as an association of plants growing to- 

 gether in the same habitat. 



150. Determining factors. Since different kinds of 

 habitats are characterized by different kinds of plant associa- 

 tions, it is important to discover the things that make habi- 

 tats different, that is, the factors that determine the char- 

 acter of a habitat in reference to -plants. It must not be 

 supposed that all the determining factors have been dis- 

 covered or that the relative importance of those that are 

 known is appreciated fully. 



The most conspicuous determining factor, and perhaps the 

 most important one, is available water, that is, water in a 

 condition to be used by plants. The range of water-supply 

 may be said to extend from 100 per cent, in which case the 

 plants would be submerged, to 5 or even 2 per cent, in which 

 case the habitat would be called a " desert." Plants differ 

 as to the amount of water they must have, and therefore a 

 plant that needs a habitat with at least a 50 per cent water- 

 supply could not live in the habitats with a less supply, and 

 might not be able to endure a much greater supply. In this 

 way, the possible range of water-supply may be thought of as 

 a violin string, which can be made to produce many different 

 tones, and each " tone " in our range of water-supply stands 

 for a different habitat and a different plant association. 



. Another determining factor is the temperature, and every 

 one knows that some plants need more heat than others. The 

 range of temperature in which plants can work may be stated 

 roughly as extending from 120 F. to 32 F. (freezing point). 

 Of course many plants can endure lower temperatures, for 

 they survive the winters, but the temperature that deter- 



