FACTORS OF ENVIRONMENT. 473 



have such a direct action on it. Snow and ice also, in the form 

 of glaciers, destroy vegetation, and on the other hand grind up 

 rocks and assist in soil formation. 



917. Leaves and other plant remains form a mulch or pro- 

 tection cover, which is especially abundant in the forest. This 

 protects the roots from extreme cold, lessens radiation, con- 

 serves moisture, etc. 



918. Living plant covers. The highest development of cover 

 by living plants is seen in the forest, and the result is shown in 

 the development of shade plants which are protected from ex- 

 cessive light and heat, while the air of the forest is more humid. 

 The under plants in a forest are also provided with better pro- 

 tection from cold. The water from rainfall is conserved, and 

 the snow melts more slowly, thus giving a more uniform distri- 

 bution of water throughout the season and lessening the danger 

 from freshets and floods. The cushions or carpets of moss, 

 formed in some places, conserve moisture. They take little 

 moisture from the ground, since the moss cushion acts some- 

 thing like a sponge to absorb and hold water from rainfall or 

 from atmospheric moisture. 



919. Chemical conditions of the ground or water. Chemical 

 constituents of soil. The chemical constituents of the soil are 

 derived from several sources, especially from solutions of eroded 

 and dissolved rock formations, from the decaying animal and 

 plant remains, from certain gases taken in solution in rain-water, 

 from salts of sea-water, etc. Since the geological formations of 

 the earth differ in their constituents, and the bulk and kinds of 

 plants and animal remains differ, and also since forces of erosion 

 and decay, and so on, vary according to certain conditions, the 

 soil varies greatly over the face of the earth and even over small 

 areas. According to investigations on certain of the higher 

 plants, ten elements are necessary for most plant growth, as 

 follows: oxygen, hydrogen, carbon, nitrogen, phosphorus, sul- 

 phur, iron, potassium, calcium, and magnesium. If any one of 

 these is lacking in suitable quantity, the plants become sickly. 

 For most green plants, most of the carbon and much of the 



