RELATIONS OF PLANTS TO EACH OTHER 195 



them much colder than the near-by hill-tops, and 

 these valleys are much warmer in the daytime than 

 the hills. 



236. The soil and plants. The chemical char- 

 acter of the soil with respect to the kind and 

 amount of food it contains, and its physical qualities 

 due to its coarseness or fineness, and power to hold 

 water, as well as its heating properties, influence 

 the communities growing upon it. The principal 

 soils are rock, sand, lime, humus or loam, and clay. 

 Beside the soil itself, the coverings of snow, leaves, 

 or living plants are of value to the community. 



237. Light and plants. Light is necessary for 

 all green plants, but some are able to make use of 

 rays of less intensity than others, and have altered 

 the structure of their entire bodies to suit this adapta- 

 tion. The species which can use the weaker light 

 usually stand under those that require the full blaze 

 of the sun, in the shadow of rocks, on northern 

 slopes, or if they are aquatic species they live at 

 some depth under the surface of the water. Along 

 a sea beach the area between high and low water 

 which is thus exposed to the full force of the sun 

 part of every day is inhabited by green, brown, and 



