444 GENERAL BOTANY 



The arrangement of plants in regular zones, termed zona- 

 tion (Figs. 287 and 288), in regions where one association is 

 being replaced by another, is often very evident, particularly 

 on the borders of streams, ponds, and lakes. In some such 

 instances the zones are very distinctly marked, while in others 

 they merge gradually into one another, creating tension lines 

 of great competition where two kinds of vegetation struggle 

 for supremacy. 



General instability of vegetation. The changes noted above in 

 new plant associations, due to migration, invasion, and competi- 

 tion, are far more general in their nature than is usually supposed. 

 The great numbers of seeds formed by each plant species in 

 nature, and the admirable devices developed for their dissemi- 

 nation, result in a wide annual sowing of the seeds of new species 

 in every old plant association. If the young plants which spring 

 from these seeds are better adapted to the environment in which 

 they chance to spring up than the existing species, the new- 

 comers will gradually drive out the older inhabitants and become 

 a new element in the population of the old association. Familiar 

 instances of such changes are seen 'in the invasion of a lawn by 

 dandelions, and of roadsides and cultivated fields by weeds. 

 Old associations are thus constantly changing in the kinds of 

 species constituting them. Another p'otent cause for far-reaching 

 changes in existing vegetation is the gradual change going on in 

 the surface features of the earth, caused by elevations and sub- 

 sidences of the earth's crust and by water erosion. Geologists 

 tell us that the continents are slowly but surely being leveled 

 off by the wearing down of the hills and by the deposit of the 

 eroded soil in the valleys by water action (Fig. 289). By these 

 processes hills and slopes are denuded, new and barren cliffs are 

 formed, and existing plant populations are covered and destroyed 

 in the valleys and on flood plains. These constant readjustments 

 initiate new plant associations, which form the first of a series of 

 plant populations, followed by successions such as those described 

 above. What is known as vegetation is therefore in constant 

 flux and change, although the individual plants which comprise 

 it are themselves immobile. 



