FORMS OF VEGETATION. 3 11 



climates. These may be reckoned as the average, or mean, 

 plants, and are therefore called technically mesophytes. 



420. A second set of conditions is characterized by de- 

 ficient water supply throughout the year, the amount of water 

 present in the soil often being less than 10$. Such regions 

 may be considered as regions of continuous drought. The 

 plants adapted to these conditions are known as drought 

 plants, or xerophytes. 



421. A third set of conditions, prevailing over compara- 

 tively limited regions, is characterized by an excess of salts in 

 Ike soil or water. These salts are chiefly sodium chloride 

 (NaCl, common salt), gypsum (CaSOj, and magnesium 

 chloride (MgCl). Plants which can live under these condi- 

 tions are known as salt plants, or halophytes. 



422. A fourth set of conditions is characterized by an 

 excess of water. The plants grow wholly or partly surrounded 

 by water, or their roots are imbedded in a soil supersaturated 

 with water, that is, containing at least 8o$. Such plants are 

 called water plants, or hydrophytes. 



It will be noticed that the first three groups, namely, meso- 

 phytes, xerophytes, and halophytes, are essentially land plants 

 in distinction from the fourth group, which are water plants. 



