Water Relations of Pocosin or Bog Shrubs.

The pocosins, or broad-leaved evergreen shrub bogs, constitute one of the most distinctive plant communities of the southeastern coastal plain. They occupy flat, poorly-drained areas where the soil is frequently water-logged for long periods of time, but where surface water seldom stands to a depth of more than a few inches, and then only for comparatively short periods. The water relations of the plants growing in pocosins present an interesting problem to the plant physiologist because structurally the leaves of many of the species resemble those of plants growing in a dry habitat rather than those of plants of wet places. The shrubs have simple, more or less elliptical leaves which are rather thick, leathery, and heavily cutinized. The leaf structure is of a type generally believed to have a low rate of transpiration, but there is never a lack of soil water except during severe summer droughts. Though the problem of the water relations of bog plants has long been a subject for discussion, all of the earlier workers confined their investigations to northern bogs. SCHIMPER (25) was among the first to notice the discrepancy between leaf structure and habitat. He advanced the theory that bogs are physiologically dry, the absorption of water being hindered by the presence of certain humic acids and soluble salts released during decomposition of the organic matter in the soil. He believed that the structure of the bog sclerophylls materially aided in preventing excessive water loss from plants whose root systems were unable to absorb much water because of the physiological dryness of the habitat. This theory of physiological drought was more or less generally accepted. Most of the earlier workers merely speculated as to the cause of the alleged physiological drought, however, and failed to present much real experimental data on the actual water relations of the plants. A number of causes of physiological drought have been suggested. Low soil temperature was regarded by SCHIMPER (25), WARMING (30), and TRANSEAU (27) as a factor in northern bogs. DAVIS (13) and BURNS (3) ascribed the "xerophytic nature" of bogs to the water-retaining power of the peat and to periodic droughts. TRANSEAU (27), CLEMENTS (6) , and BERGMAN (2) regarded the lack of oxygen in bog soils as the controlling, or at least the primary, factor in physiological dryness. The importance of toxic compounds was first suggested by LIVINGSTON (17, 18) and has been advocated by DACHNOWSKI (9, 10, 11) and RIGG (23). These so-called toxic compounds are supposedly the result of incomplete decomposition of organic substances in a soil deficient in oxygen. The hypothesis of physiological drought, originally developed in connection with bogs of cool climates, has been extended to explain the vegetative