Naturally the temperature of the soil always bears a due relative proportion to that of the air. When the temperature of the air decreases, that of the soil also decreases, but very slowly; and when the atmospheric heat increases, that of the soil also gradually rises. Bottom heat, or heat applied below the roots of plants, is the artificial mode of imitating this proceeding of nature in our hothouses, and other structures of that kind. If the temperature of the soil be too cold in proportion to the temperature of the atmosphere, the roots are not stimulated sufficiently to imbibe food as fast as it is required by the branches and foliage, and as a consequence the leaves or fruit will fall or wither. On the other hand, if the temperature of the soil be too great in proportion to that of the atmosphere, the roots absorb food faster than it can be elaborated by the branches and leaves, and as a consequence, over luxuriant shoots, and an extra development ofleaves, are caused instead of blossoms, and a healthy progress in all the parts.

Every plant obviously will have a particular bottom heat most congenial to it. Plants growing in open plains, as at the Cape of Good Hope, will require a higher bottom heat than those growing in the shade of the South American forests, though the temperature of the air out of the shade may be the same in each country. That gardener will succeed in exotic plant-culture best, who among his' other knowledge has ascertained the relative temperature of the air and soil in which any given plant grows naturally. At present, such information from actual observation is not obtainable, but it is not so difficult to ascertain the maximum and minimum temperature of the air of a country; and this being obtained, the gardener may adopt this as a safe rule. Let the bottom heat for plants of that country be always 5° higher than the average temperature, or if the average maximum temperature of the air only be known, let the bottom heat be less by 10° than the maximum temperature of the air.

Dr. Lindley very justly observes upon this subject, that "we have doubtless much to learn as to the proper manner of applying bottom heat to plants, and as to the amount they will bear under particular circumstances. It is in particular probable, that in hot-houses plants will not bear the same quantity of bottom heat as they receive in nature, because we cannot give them the same amount of light and atmospheric warmth; and it is necessary that we should ascertain experimentally whether it is not a certain proportion between the heat of the air and earth that we must secure, rather than any absolute amount of bottom heat.

"It may also be, indeed it no doubt is, requisite to apply a very high degree of heat to some kinds of plants at particular seasons, although a very much lower amount is suitable afterwards; a remark that is chiefly applicable to the natives of what are called extreme climates, that is to say, where a very high summer temperature is followed by a very low winter temperature; such countries are Persia, and many parts of the United States, where the summers are excessively hot, and the winter's cold intense. The seeming impossibility of imitating such conditions artificially, will probably account for many of the difficulties we experience in bringing certain fruits, the Newtown pippin, the cherry, the grape, the peach, and the almond, to the perfection they acquire in other countries." - Theory of Horticulture.