276 ELEMENTARY STUDIES IN BOTANY 



example, compare the number of individual plants in a well- 

 kept corn-field, in which we are trying to cultivate as many 

 individuals of one kind as possible, with the number of in- 

 dividuals (population) on an equal area in nature. In the 

 former case, the individuals stand well apart and are com- 

 paratively few ; while in the latter case they stand thickly 

 together and are many times more numerous. In both 

 cases, the plants are " doing well." The Chinese have taken 

 advantage of this fact in their cultivation of plants, raising 

 two or three different crops simultaneously on the same area, 

 and thus increasing the total population and of course 

 the total yield. In cultivating only one kind of plant at 

 a time on an area, therefore, we are reducing the possible 

 plant population to its minimum. 



152. Succession of plant associations. The most im- 

 portant fact in reference to a plant association is that it is 

 not permanent on a given area. In general, when a plant 

 association lives for a time upon an area, that area becomes 

 increasingly unfavorable to it, until gradually it is succeeded 

 by another plant association. Almost any plant associa- 

 tion finally makes conditions unfit for itself, and at the same 

 time more fit for some other association. This succession 

 of plant associations may be illustrated by the succession 

 of human communities. Pioneer conditions bring together 

 a characteristic association of individuals, but the conditions 

 do not remain pioneer, and become favorable for another 

 association of individuals, and this kind of succession may 

 go on, until the series of associations can be traced from the 

 pioneer association to the metropolitan association. This 

 means that each plant association can reveal the succession 

 of associations that preceded it and also the succession of 

 associations that will succeed it. In other words, the most 

 important thing about a plant association is the history and 

 prophecy it contains. 



It is evident that there may be many kinds of succession, 



