INFLUENCE OF WATER ON PLANT-LIFE 29 



1. Hygrophytes (Gr. hygros, moist ; phyte, plant), 

 plants adapted to very moist conditions. 



2. Xerophytes (Gr. xeros, dry), plants adapted to live 

 in a dry soil or under conditions unfavourable to their 

 development, at least during a part of the year. The 

 conditions may aU be ultimately referred to the quan- 

 tity, availability, and usefulness of the water-supply 

 (Chapter IV.). A wet soil is dry if the plant can absorb 

 none of the water. 



3. Mesophytes (Gr. mesos, middle, intermediate), plants 

 occupying an intermediate position with regard to water. 



1. Hygrophytes. 



True hygrophytes are plants which live in places always 

 moist, and which, during the whole year and the whole 

 of their lives, exhibit characters adapted to moist con- 

 ditions. The extreme hygrophyte Hves in an atmosphere 

 saturated with moisture. It cannot transpire, but the 

 transpiration-current is kept going by the elimination of 

 water in Uquid drops through hydathodes (p. 27). More- 

 over, such plants can only live in regions where no \m- 

 favourable season intervenes to disturb their develop- 

 ment. Winter-cold means drought even for a hygrophyte 

 (p. 64). For this reason they are only found in the wet 

 parts of the Tropics, where there is no dry season and 

 no winter. They are evergreen, and live in the shade 

 of trees or dripping rocks. Outside the Tropics they are 

 only found in very damp, shady places where frost is 

 imknown. In the British Isles the nearest approach to 

 these conditions is found in the extreme south-west of 

 Ireland, and the plants which, in their characters, most 

 nearly resemble true hygrophytes are the Fihny Ferns. 

 The KiUamey fern, for example, is a plant with delicate 

 evergreen leaves, and lives in wet, sheltered crannies 

 in the rocks near the Lakes of Killamey. Elsewhere 

 in this country they can only be reared ia glass cases, 

 which are shaded from the sun, and in which the air 

 is kept constantly saturated with moisture. 



The leaves of many of our marsh and wet-meadow 

 plants exhibit hygrophihus (Gr. hygros, moist ; phileo, 

 I love) characters in summer ; but as they perish at the 



