That pairs of closely related plants may differ markedly in soil-reaction preference has long been recognized by European ecologists, beginning with Kerner (1869), although before the development of modern theories of reaction these were classed as calcicoles or calciphiles on the one hand, and oxylophytes or calciphobes on the other. The best summary of the earlier work is that by Warming ('o9). Atkins ('22) reported the results of actual reaction tests on such pairs in England. The presence of similar pairs of species in North America has been pointed out by Fernald ('07, '14, 'i6), but many American ecologists seem to have ignored this fact, and have stated or implied that in studies of the relation of plants to environmental conditions precise identification of species or varieties is unnecessary. Early in the course of the writer's studies on the relation of soil reaction to plant distribution in eastern North America it became evident that careful discrimination of species, varieties, etc., is essential, since sets of closely related plants showing divergent soil-reaction preferences prove to be not infrequent in this country. Some of these have already been referred to by the writer ('I8, '20-I, '20-2, '20-3, '21-1, '21-2, '22-I, '24, '25-2, '26-I, and '26-2), but in the present paper they are discussed more fully, and attention is called to a number of additional ones, upon all of which it has been possible to carry out enough soil-reaction tests for their allocation to be reasonably certain. In all, thirty groups are covered. The soil-reaction terms used are those previously proposed (Wherry, 'i9, '22-3), although it seems well to simplify their application somewhat, as follows ('26-3):
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