440 GENERAL BOTANY 



association. In other cases, as we shall learn later, the herbaceous 

 plant association may be replaced in time by a shrub or a forest 

 plant association. Many plants, also, when once established in a 

 given locality spread by vegetative means and gradually drive 

 out other competitors. This is notably true of plants with run- 

 ners, stolons, rhizomes, and other underground parts, already dis- 

 cussed under vegetative reproduction in the first pa-rt of jthe 



FIG. 286. The pond lily, an aquatic with floating leaves and submerged stems 



text. In the end a new plant association will be formed, composed 

 of plants adapted to the conditions of soil and climate which 

 obtain in the given region. The above sketch of the main factors 

 involved in the making of a new plant association on a denuded 

 tract is probably a fairly accurate picture of the manner in which 

 the existing plant associations which constitute the earth's vege- 

 tation have arisen. 



Succession. The term succession involves the idea of replac- 

 ing the plants comprising a given association by a new plant 

 population which invades and finally occupies the ground for- 

 merly held by the old association. This conception can be most 



