. Research methods in ecology. Plant ecology. 122 THE PLANT increase of air-spaces is correlated with reduction of the pahsade, and a decided increase in the sponge. An increase in water supply is indicated by the absence of storage tissues, and the reduction of the vascular system, which, however, is more closely connected with a diminished need for mechanical support. 166. Plant types. The necessity for decreasing or increasing water loss in compensation of the water supply has made it possible to distinguish two fundamental groups of plants upon the twofold basis of habitat and struc- ture.


. Research methods in ecology. Plant ecology. 122 THE PLANT increase of air-spaces is correlated with reduction of the pahsade, and a decided increase in the sponge. An increase in water supply is indicated by the absence of storage tissues, and the reduction of the vascular system, which, however, is more closely connected with a diminished need for mechanical support. 166. Plant types. The necessity for decreasing or increasing water loss in compensation of the water supply has made it possible to distinguish two fundamental groups of plants upon the twofold basis of habitat and struc- ture. These familiar groups, xerophytes and hydrophytes, represent two extremes of habitat and structure, between which lies a more or less vague, intermediate condition represented by mesophytes. These show no char- acteristic modifications, and it is consequently impossible to arrange them in subgroups. Xerophytes and hydrophytes, on the other hand, exhibit marked diversity among themselves, a fact that makes it desirable to recognize subgroups, which correspond to fundamental differences of habitat or adaptation. It is hardly neces- sary to point out that these types are not sharply defined, or that a single plastic species may be so modified as to ex- hibit several of them. The extremes are always clearly defined, however, and they. Fig. 32. Mesophyll of Pedicularis procera (chresard, 15^, light, 1). X130. indicate the specific tendency of the adaptation shown by other members of the same group. 167. Xerophytic types. With the exception of dissophytes, all xero- phytes agree in the possession of a deep-seated root system, adapted to withdraw water from the lower moist layers, and to conserve from loss from the upper dry layers. Reservoirs are developed in the root, however, in relatively few cases. The stem follows the leaf more or less closely in its modification, except when the leaf is greatly reduced or disappears, in which event the stem exhibits peculiar adaptations. While


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Keywords: ., bookcentury1900, bookdecade1900, booksubjectplantec, bookyear1905