CHAPTER VII — PLANT ASSOCIATIONS 



Definition. — In the preceding chapters, plants have been considered 

 as individuals having certain relations to their physical surroundings, 

 or to each other. While plants sometimes occur as isolated individuals, 

 they are associated far more commonly in more or less definite groups. 

 So true is this that when one who is familiar with nature sees a given 

 species in the field, he comes almost instinctively to look for other species 

 that he has seen associated with it. If he is observant, he looks a 

 step further arid finds that these associated species also are associated 

 with a definite kind of habitat. For example, pitcher plants, sundews, 

 cranberries, and peat moss grow together in imperfectly drained swamps 

 known as bogs or moors ; beach peas, sea rocket, and beach grass 

 grow together on sandy coasts; beech, maple, beech fern, and beech- 

 drops grow together in mesophytic forests. A group of plants in its 

 entirety occurring in a common habitat is known as a plant association. 

 Sometimes the association of specific plants is obligate, as in the case 

 of the beechdrops, which grows parasitically on the beech, while the 

 beech in turn appears to require certain fungi in the soil. More 

 commonly, however, the association is purely facultative. Pitcher 

 plants and sundews or maples and beeches grow together, because 

 they thrive in similar conditions; so far as is known, the presence 

 or the absence of one is a matter of no particular consequence for 

 the other, except as it occupies or leaves a certain amount of space. 



The kinds of associations. — Plant associations have been variously 

 classified, the simplest grouping being based on the water relation, and 

 the large divisions being termed hydropliylic, mesophytic, and xerophytic, 

 while these in turn are subdivided into various groups of associations. 

 This classification, though advantageous because of its ready applica- 

 tion, has the great disadvantage of grouping together associations that are 

 entirely unrelated in origin, such as those of bogs and ordinary swamps, 

 while separating closely related associations, such as those of bogs and 

 of the coniferous forests into which they commonly develop. Though 

 it is more difficult to apply, there are many advantages in a genetic classi- 



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