FORMS OF VEGETATION. 311 



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

 plants, and are therefore called technically mesophyies. 



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 

 the soil or water. These salts are chiefly sodium chloride 

 (NaCl, common salt), gypsum (CaS0 4 ), 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 80$. 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. 



