468 RELATION TO ENVIRONMENT. 



enable them to retain the small amount they can absorb from 

 the soil and give it off slowly. Otherwise they would dry out 

 by evaporation and die. Some of the structures which enable 

 xerophytic plants to withstand the conditions of dry climate 

 and soil are lessened leaf surface, increase in thickness of leaf, 

 increase in thickness of cuticle, deeply sunken stomates, compact 

 growth, also succulent leaves and stems, and in some cases loss 

 of the leaf. Evergreens of the north temperate and the arctic 

 regions are xerophytes. 



907. Hydrophytes, These are plants which grow in fresh 

 water or in very damp situations. The leaves of aerial 

 hydrophytes are very thin, have a thin cuticle, and lose water 

 easily, so that if the air becomes quite dry they are in danger of 

 drying up even though the roots may be supplied with an abun- 

 dance of water. The aquatic plants which are entirely submerged 

 have often thin leaves, or very finely divided or slender leaves, 

 since these are less liable to be torn by currents of water. The 

 stems are slender and especially lack strengthening tissue, since 

 the water buoys them up. Removed from the water they droop 

 of their own weight, and soon dry up. The stems and leaves 

 have large intercellular spaces filled with air which aids in aera- 

 tion and in the diffusion of gases. Some use the term hygrophytes. 



908. Halophytes. These are salt-loving plants. They grow 

 in salt water, or in salt marshes where the water is brackish, 

 or in soil which contains a high per cent of certain salts, for example 

 the alkaline soils of the West, especially in the so-called "Bad 

 Lands" of Dakota and Nebraska, and in alkaline soils of the 

 Southwest and California. These plants are able to withstand 

 a stronger concentration of salts in the water than other plants. 

 They are also found in soil about salt springs. 



909. Tropophytes.* Tropophytes are plants which can live as 

 mesophytes during the growing season, and then turn to a 

 xerophytic habit in the resting season. Deciduous trees and 

 shrubs, and perennial herbs of our temperate regions, are in 

 this sense tropophytes, while many are at the same time mesophytes 



* Term used by Schimper. 



