Schimper used the term xerophytes to include plants which live in soils which are physiologically dry, and the term hygrophytes those which live in soils which are physiologically wet or damp. Schimper recognized that the two classes are connected by transitional forms, and that it is useless to attempt to give the matter a statistical basis.
An interesting special case of hygrophytes is seen with regard to plants which live in the shade of forests.
Thus all existing hygrophytes (excepting the Algae) are considered to have been derived from land-plants which have adapted themselves to a watery habitat.
Whilst Schimper objected to the constitution of a special category, such as mesophytes, to include all plants which are neither pronounced xerophytes nor pronounced hygrophytes, he recognized the necessity of a third class in which to place those Schuuw, Grundtraek til en almindelig Plantegeografie (Kjbbenhavn, 1822); German trans., Grundzuge einer allegemeinen Pflanzengeographie (Berlin, I 823).
Tropophytes.Plants which are hygrophytes during some favorl part of the year and xerophytes during the rest of the year; ~., deciduous trees and shrubs, deciduous herbaceous plants with iderground perennating organs, and annuals and ephemerals.