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Habitats of the land biocycle are differentiated primarily with reference to the degree of saturation of the soil by water. So important is this factor that plants may be classified according to their water requirements. The terminology associated with the water factor is built upon three simple prefixes of Greek roots: xero-, dry; hydro-(hygro), wet; and meso-, intermediate or middle. Those plants, which grow in dry habitats, are xerophytes; those, which grow in water or in wet habitats, are hygrophytes (hydrophytes); those of habitats of an intermediate degree of wetness and relatively uniform water availability are mesophytes.

The xerophytes are highly tolerant of drought and can survive in habitats, which dry quickly following rapid drainage of precipitation, for example, on sand dunes, beaches, and bare rock surface. The plants typical of dry climates (deserts) are also xerophytes; cactus is an example. The hygrophytes are tolerant of excessive water and may be found in shallow streams, lakes marshes swamps, and bogs; and example is the water lily. The mesophytes are found in upland habitats in regions of ample rainfall. Here the drainage of precipitation is good and moisture penetrates deeply where it can later be used by the plants. On such upland locations the soil may be of intermediate texture-not too coarse or too fine-and is usually thickly and uniformly present.

Certain of the climatic regimes, such as the tropical wet-dry regime (including monsoon climates) and the continental regime which is moist but not too cold, have a yearly cycle with one season in which water is unavailable to plants because of lack of precipitation or because the soil water is frozen. This season alternates with one in which there is abundant water. Plants adapted to such regimes are termed tropophytes, from the Greek word trophos, meaning change, or turn. Tropophytes may meet the impact of the season of unavailable water by dropping their leaves and becoming dormant. when water is again available, they leaf out and grow at a rapid rate. Trees and shrubs, which seasonally shed their leaves, are said to be deciduous; in distinction with evergreen trees which retain most of their leaves in green state throughout the year.

Next: Water needs of plants