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The Individualistic Concept of the Plant Association

H. A. Gleason
The American Midland Naturalist
Vol. 21, No. 1 (Jan., 1939), pp. 92-110
DOI: 10.2307/2420377
Stable URL: http://www.jstor.org/stable/2420377
Page Count: 19
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The Individualistic Concept of the Plant Association
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Abstract

1. The ordinary processes of migration bring the reproductive bodies of a single plant or a species of plant into many places. 2. The ordinary processes of migration bring the reproductive bodies of various plants into the same place. 3. Of the various species which reach one spot of ground, the local environment determines which may live, depending on the individual physiological demands of each species separately. 4. On every spot of ground, the environment varies in time, and consequently the vegetation varies in time. 5. At any given time, the environment varies in space, and consequently vegetation varies in space. 6. A piece of vegetation which maintains a reasonable degree of homogeneity over an appreciable area and a reasonable permanence over a considerable time may be designated as a unit community. Within such an area and during such a period similarity in environmental selection tends toward similarity in vegetation. 7. Since every community varies in structure, and since no two communities are precisely alike, or have genetic or dynamic connection, a precisely logical classification of communities is not possible. Although a summary is generally considered to close a discussion, it nevertheless seems desirable to indicate very briefly the relation of the individualistic concept to other current philosophies of the plant community. The individualistic concept is totally at variance with the idea that the association is an organism, represented by many individuals, and also does not admit an analogy or homology between the species and the association. While affirming the existence of definite communities, characterized by reasonable uniformity over a considerable area terminated by a definite boundary, the concept denies that all vegetation is thus segregated into communities. The concept is by no means opposed to the recognition of the synusia, or union, defining it as a group of plants whose physiological demands are so similar that they are regularly selected by the same environment and consequently regularly live together. If classification of communities on the basis of floristic resemblance is rejected, and if broader areas of vegetation are searched for characters indicating genetic or dynamic similarity, these are first found in the broad floristic group now known as a biome. The plants of a biome, living together now as their ancestors have lived together and evolved together in the past, represent an environmental selection of a broad type, while lociation and faciation within the biome indicate the variability of the environment and the irregularity of migration.

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