_Heat and Moisture_ are the main governing conditions of plant growth. In a preceding chapter the relations of soils and plant growth to water have been considered; in the present one the relations of both moisture and heat to soils and plants will be discussed; and to do this intelligibly to those not making a specialty of such studies, it becomes necessary to introduce, first, a summary consideration of the subject of climate. Climatic conditions control, and to a great extent determine, the industrial pursuits of every country; all the more so as the rapid communication and transportation by means of modern appliances now brings every part of the globe in competition with every other. The question is not now what it may be intrinsically possible to do under certain climatic and geographical conditions, but whether these things can be done with a reasonable prospect of profit and commercial success, in competition with other countries offering more or less of similar possibilities. While it is true that the cost of labor frequently enters most heavily into such problems, yet favorable or unfavorable climatic or soil-conditions may in many cases turn the scale. Thus the high price of labor and fuel on the Pacific coast of the United States would at first blush seem to render competition with Europe and the East in the production of beet sugar commercially impossible; yet exceptionally favorable climatic and soil-conditions concur to turn the scale in favor of California at least, so as to have placed that state at the head of the sugar-producing states of the Union. A general understanding of the climatic conditions which concern the United States more or less directly, is therefore needful to an appreciation of their agricultural possibilities. _Climatic Conditions._—The factors usually mentioned as constituting climate are temperature, rainfall and winds. Since the latter two factors, however, are themselves merely the result of _heat_ conditions, it is proper to discuss from the outset the origin and mode of action of heat. TEMPERATURE.