PLANTS MODIFIED BY THEIR SURROUNDINGS 167 



meadow, along a roadside, or at the edge of a pond. Any one 

 familiar with the country knows instinctively that we find certain 

 plants, and those plants only, living together under certain condi- 

 tions. For example, the wild columbine, certain ferns, and mosses, 

 and other shade, moisture, and rock-loving plants are found to- 

 gether on rocky, shaded hillsides. We should not think of looking 

 for daisies and buttercups there any more than we should look for 



Plant societies near a pond. Notice that the plant groups are arranged in zones 

 with reference to the water supply, the true mesophytes being in the background. 



the marsh marigold (Caltha palustris) or the pickerel weed (Ponte- 

 deria cor data) in a dry and sunny field. 



Plants associated under similar conditions, as those of a forest, 

 meadow, or swamp, are said to make up a formation, and a plant 

 formation is brought about by the conditions of its immediate 

 surroundings, the habitat of its members. If we investigate a plant 

 formation, we find it to be made up of certain dominant species of 

 plants; that here and there definite communities exist, made up 

 of groups of the same kind of plants. We can see that every one of 

 these plant groups in the society evidently originally came from 

 single individuals of species which thrive under the peculiar condi- 

 tions of soil, water, light, etc., that we find in this spot. These 



