230 OUTLINES OF PLANT LIFE. 



to people in the fertile regions of temperate climates. These 

 may be reckoned as the average, or mean, plants, and are 

 therefore called technically mesophytes. 



319. A second set of conditions is characterized by defi- 

 cient water supply throughout the year, the amount of water 

 present in the soil often being less than loj^. Such regions 

 may be considered as regions of continuous drought. The 

 plants adapted to these conditions are known as drought 

 plants, or xerophytes. 



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

 tively limited regions, is characterized by an excess 0/ salts in 

 the soil or water. These salts are chiefly common salt, gypsum, 

 and magnesium chloride. Plants which can live under these 

 conditions are known as salt plants, or halophytes. 



321. A fourth set of conditions is characterized by an 

 excess of water. The plants grow wholly or partly sur- 

 rounded by water, or their roots are embedded 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. 



322. Summary, — In order to exist at all, plants must 

 adapt themselves to the places in which they live. Compe- 

 tition for light, water, and soil room is intense because of the 

 number of individuals. Competition of better adapted kinds 

 may exterminate or force migration. The factors to which 

 plants must adjust themselves are many. Each factor is more 

 or less variable and different factors liiay be combined in any 

 ratio, producing almost infinite diversity. Plants differ, 

 chiefly because of this diversity of conditions under which 

 they grow. For convenience the water relation is used to 

 group plants into four vegetation forms, mesophytes, xero- 

 phytes, halophytes, and hydrophytes. 



