326 A TEXT-BOOK OP BOTANY 



189. Light. All green plants cannot have an equal 

 amount of light, and some have learned to live with a less 

 amount than others. In a general way this difference is 

 recognized in the terms light-plants and 'shade-plants, and 

 it permits plants to grow in strata. For example, in a 

 forest association the tall trees form the highest stratum; 

 below this there may be a stratum of shrubs, then tall 

 herbs, then low herbs, then mosses and lichens growing 

 close to the ground. If a forest is cleared, the remaining 

 plants of the association are very much affected; and if a 

 forest encroaches upon another association it is sooner or 

 later destroyed. The development of the vernal habit 

 in connection with deciduous forests, which was described 

 in 27, is a means by which certain plants avoid the forest 

 shade and secure the forest soil. 



190. Wind. In regions of strong and more or less 

 continuous wind, as near the seacoast, around the Great 

 Lakes, and on the prairies and plains, this condition has 

 much effect upon the character of the plants. Wind is a 

 great drying agent, and increases the loss of water from 

 plants by transpiration ( 15), so that plants exposed to it 

 must be able to check transpiration. 



191. The great groups of associations. For convenience, 

 the very numerous plant associations are grouped on the 

 basis of their water-supply. Such a classification is not a 

 natural one, for no single condition determines an asso- 

 ciation; but for general purposes it serves well to introduce 

 the associations to observation. On this basis there are 

 three great groups of associations, as follows: 



(1) Hydrophytes. The name means "water-plants," 

 and applies to those associations with an abundant water- 

 supply, growing in water or in very wet soil. 



(2) Xerophytes. The name means "drought plants," 

 and applies to those associations with a scanty water-sup- 

 ply. True xerophytes are exposed to dry soil and air. 



