THE ECOLOGY OP PLANTS. 415 



same amount of light or of shade, etc. Such groups of 

 plants, growing together in a sort of community, may be 

 called Plant Associations. 



There are four main ecological plant- types. At one 

 extreme we have plants adapted to life in ponds, rivers, 

 ditches, growing partly or entirely submerged in water. 

 These true water-plants are said to be hydrophilous 

 (i.e. " water-loving "), and are termed Hydrophytes. 

 Plants which grow in marshes or swampy ground, on 

 river-banks or on ditch- sides, or in damp shady woods, are 

 said to be hygrophilous (" moisture-loving ") plants, or 

 Hygrophytes. 



At the other extreme we have plants which are adapted for 

 life under conditions of " physiological " drought that is, 

 under such conditions that it is necessary to check or greatly 

 reduce the rate of transpiration, or to store up water, or to 

 do both, because the water supply is scanty or runs through 

 the soil too quickly for the roots to absorb much of it, or 

 because the water cannot be absorbed freely on account of 

 the presence of excessive amounts of dissolved salts or of 

 peaty matter, or because the plant grows in places exposed to 

 high drying winds, or in cold soil, etc. 



A little reflection will show that any of these causes 

 will lead to the development of adaptations for reducing 

 transpiration or for storing the scanty or precarious 

 supply of water which the plant can absorb. In other 

 words, such plants will be xerophilous ("drought-loving") 

 plants, or Xerophytes. Xerophytes occur on heaths and 

 moors, also in the boggy places which accompany them ; 

 on high mountains with an Alpine flora; in sandy and 

 gravelly places; on chalk downs; on the sea-coast (cliffs, 

 rocks, sandy beaches, sand-dunes, salt-marshes) ; and in dry 

 woods or plantations. 



Between xerophytes and hygrophytes we find a great many 

 intermediate forms (Mesophytes), comprising, in fact, the 

 majority of British plants, which grow in meadows, pastures, 

 cultivated fields, and in damp woods and plantations of 

 deciduous trees (Beech, Oak, Birch, etc.). These plants show 

 no decided xerophilous or hygrophilous characters, though 

 some approach xerophytes in form and structure and others 

 approach hygrophytes. 



