274 ELEMENTARY STUDIES IN BOTANY 



mines a plant association is the working temperature. The 

 range of temperature may be likened to a second violin 

 string that can produce different tones, each temperature 

 " tone " standing for a different plant association. 



These two determining factors (water and temperature) 

 introduce the idea of combinations of factors. For example, 

 two violin strings make possible a far greater variety of tones 

 when used in combination than when used singly. In the 

 same way the combination of two determining factors mul- 

 tiplies plant habitats and associations. If there is a com- 

 bination of maximum water-supply and maximum tempera- 

 ture, the plant association will be a tropical jungle ; but 

 if there is a combination of minimum water supply and 

 maximum temperature, the habitat will be a desert. This 

 indicates the numbers of different plant associations that vari- 

 ous combinations of factors make possible. 



Other determining factors, such as soil, light, wind, etc., 

 have been studied, but they do not need to be discussed here. 

 They suggest, however, that the different kinds of plant as- 

 sociations may be very numerous, for if a violin with two 

 strings can make possible a large number of combinations, 

 what must be the possible number of combinations when the 

 violin has six or more (probably many more) strings? This 

 simply means that each plant association has an individuality 

 of its own, just as each town or city has its own individuality. 

 Therefore, wherever one goes, he meets with new kinds of 

 plant associations, just as he meets with new kinds of cities 

 wherever he travels. 



151. Some features of a plant association. When a plant 

 association is visited, it may be looked upon as a community 

 whose population consists of plants. There are certain 

 general features in the community life of such a population 

 that become evident at once. 



One of the most obvious facts is that certain individuals 

 dominate and give tone to the community. For example, 



