Because they do not absorb the solar rays, _and are therefore cold to the roots of plants_. 1163. _Why are mixed soils favourable to vegetation?_ Because they contain the _elements of nutrition_ essential to the development of the vegetables, and the plants absorb from them those constituents which are necessary to their growth. 1164. _Why do farmers sow different crops in rotation?_ Because every plant takes something from the soil, and gives something back; but all kinds of plants do not absorb nor restore the elements in the same proportions. Therefore a succession of crops of one kind would soon impoverish the soil; but a succession of crops of different kinds will compensate the soil, in some degree, for the nourishment withdrawn. [Verse: "He watereth the hills from his chambers; the earth is satisfied with the fruit of thy works."--PSALM CIV.] 1165. _Why do farmers manure their lands?_ Because, as soils vary, and crops impoverish the soils, the farmer employs manure _to restore fertility_, and to _adapt the soils to the wants of the plants_ he desires to cultivate. It is remarkable that Nature herself points out to man the necessity for changing the succession of vegetable growths. When plants have exhausted the soil upon which they grow, they will push their roots far in search of sustenance, and in time migrate to a new soil, while other plants will spring up and thrive upon the area vacated. When a forest in North America is destroyed by fire, the trees that grow afterwards are unlike those that the fire consumed, and evidently arise from seeds that have long lain buried in the earth, waiting the time when the ascendancy of the reigning order of plants should cease. 1166. _Why are grasses so widely diffused throughout nature?_ Because they form the _food_ of a very large portion of the animal kingdom. They have therefore been abundantly provided. No spot of earth is allowed to remain idle long. When the foot of man ceases to tread down the path, grass immediately begins to appear; and by its universality and the hardihood of its nature, it clothes the earth as with a carpet. Many grasses, whose leaves are so dry and withered that the plants appear dead, revive and renew their existence in the spring by pushing forth new leaves from the bosom of the former ones.--_Withering's Botany._