326 A TEXT-BOOK OF 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, isa 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.—F or 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) Nerophytes.—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. 
