468 FELA TION TO ENVIKONMENT. 



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 chmate 

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

 increase in thickness of cuticle, deeplv 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 .\erophytes. 



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 

 easilv, so that if the air becomes cjuitc drv thev are in danger of 

 drying up even tliougli the roots ma_y be su])phed with an a1)un- 

 dance of water. The acj;uatic plants wliich arc entirch' submerged 

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

 since these are less lialjle to be torn \)\ currents of water. The 

 stems arc slender and especially lack strengthening tissue, since 

 the water buo\-s them up. Removed from the water thev 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 llie term // vt^ropli vies. 



908. Halophytes. — These are saltdoving plants. The\^ grow 

 in salt water, or in salt marslies wliere the water is brackish, 

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

 the alkaUne 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 ])lanls are alile to withstand 

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

 The\' are also found in snil ab(ait salt s])rings. 



909. Tropophytes.'i-— 'J'ropo])hytes are jjlants wliich can li\e as 

 meso])hvtes during the growing season, and then turn to a 

 .xeroplnlic liabil in the resting season. Deciduous trees and 

 shrubs, and perennial lierbs ol our temperate regions, are in 

 lliis sense lro])oplMles, while man)- ;ire :it tlie s:inie tiiue niesojilu'tes 



* Trrni ii.scd ]jy St hinijur. 



