CONTINUING CAUSES OF SUCCESSION 207 



pretation of the past history of vegetational changes, they are of 

 little importance in the study of current seres. 



On every part of the earth's surface that is not already level there 

 is a tendency for the action of physiographic factors to result in a 

 leveling of the land. This is brought about largely through erosion 

 from the higher places and deposit in the depressions. These proc- 

 esses are influential in causing plant successions to continue. 



By far the most important causes of succession, however, are 

 biotic. Each plant community in a sere changes the environment 

 in such a way that the habitat gradually becomes better suited to 

 the component species of the immediately succeeding stage and less 

 suited to the existing one. The processes that are concerned in the 

 biotic causes of succession may be grouped under three headings; 

 namely, invasion, competition, and reaction. Invasion means the 

 movement of plants from one area into another and their coloniza- 

 tion in the latter. The movement from one area to another con- 

 stitutes migration and the colonization of plants in a new home is 

 known as ecesis. The various means by which plants migrate were 

 discussed in Chapter XVL IVIigration, however, must be followed by 

 ecesis if the invasion of the new area is to be successfully completed. 



Most plants produce a great excess of disseminating bodies. A 

 single pigweed {AmarantJms retroflexus) has been known to produce 

 2,350,000 seeds. A single specimen of the pasture mushroom {Agari- 

 cus cam'pestns) , according to an estimate based on results obtained 

 with a counting apparatus, may produce 1,800,000,000 spores, while 

 a large specimen of the shaggy mane mushroom {Coprimis comatus) 

 (Fig. 92) may produce 5,240,000,000 spores. Some conception of 

 this immense number may be obtained from the statement that, 

 although these spores are microscopic in size, if the number from the 

 above shaggy mane mushroom were placed end to end they would 

 extend a distance of about 41 miles. But the world is full of i)lants 

 now, so that, on the average, there is a chance for only one dis- 

 seminule from each plant to grow. Thus, the great waste of seeds 

 and spores through migration to unfavorable places is not a serious 

 matter. The whole surface of the earth is bombarded with seeds 

 and spores of innumerable kinds each year but after migration is 

 completed ecesis can take place only under favorable conditions. 



Although migration often proceeds over great distances the 

 proximity of the young community to parent vegetation, or feeder 

 vegetation as it may be called, is an important factor in controlling 

 the rate at which invasion may proceed. As a rule the greater num- 



