Distribution of Plants in Nature 307 



Hills are eroded by wind and water, and their surface materials 

 are being constantly carried to lower levels. Ponds and lakes 

 are continually being filled by material that is carried into them 

 by the wind and water, or by the material that accumulates 

 through the death of the plants and animals living in the water. 

 Streams enlarge their valleys, eroding here and depositing there, 

 but constantly changing and wearing away the slopes and the 

 valley bottoms. The plant associations that exist in these vari- 

 ous physiographic situations are affected by all these changes ; 

 consequently the character of the association also changes. 



In addition to the physiographic changes, the vegetation 

 itself, through shading, brings about* changes in light, tempera- 

 ture, and moisture. Humus accumulates in the soil, increasing 

 the constancy of the water supply and affording better conditions 

 for the growth of the bacteria and fungi which improve the 

 available supply of soil salts. Animals, particularly earthworms 

 and insects, also aid in these processes. 



Habitats, then, are constantly changing; and in the course 

 of years, decades, or centuries the conditions may be so altered 

 that the kinds of plants now living in the habitat cannot survive 

 and other kinds will have taken their places. This process of 

 change in vegetation is called succession. Examples of suc- 

 cession may be seen in fields that have been abandoned and al- 

 lowed to return to a wild condition. 



In the forested regions of New England it is not uncommon to 

 see areas embracing several hundred acres, once highly cultivated, 

 but now, through abandonment, completely reverted to forest 

 again. Other examples of succession may be seen along railroad 

 cuts and fills, on sand dunes, and on sand bars and islands in 

 streams. Here the newly exposed areas are occupied by asso- 

 ciations of plants very different from the plants on areas that 

 have been covered with vegetation for 10, 20, or more years. 

 The youngest areas may contain mostly a great variety of an- 

 nual weeds; the older areas are covered with perennial herbs 



