Fruit only obeys the general laws which regulate the formation of vegetable secretions. Heat and light are unquestionably the agents, though perhaps not the sole agents, upon which all the qualities of plants depend. No art can induce the rhubarb plant to form in Europe the medicinal substances which give value to the drug in those bright and heated regions of Asia which it inhabits, nor can the tomatoes ripened in England be for a moment compared, for excellence, to those produced in the north of Africa. Among the immediate causes of the changes that occur in the secretions of fruit, heat and light being of the first importance, the peculiar qualities of fruit are imperfectly formed without them, especially in species that are natives of countries enjoying a high summer temperature. It is found, that among the effects of high temperature and an exposure to bright light, is the production of sugar and of certain flavors, and that, under opposite circumstances, acidity prevails.

Very curious results are produced by this law on plants in the same latitudes, under different circumstances of light and heat. In some parts of England, for instance, trees and plants which are natives of tropical climates, often remain in the open ground through the winter without injury. Oranges, citrons, myrtles, camellias, magnolias, the Mexican agave, &e., require no protection from frost, and, in sunny exposures, are grown in the open air; yet the above fruits are difficult to ripen under the most favorable circumstances of position. The grape rarely ripens, while currants are acid, and only gooseberries and strawberries attain perfection; the apple, hardy as it appears in the American climate, rarely comes to perfection in positions of the average exposure, owing to the low temperature and humidity of the summer there, in contrast with the high temperature and freedom from sensible moisture here. The spread of wheat and the better cereals, is of recent date in the British islands, and is due only to the great care and superior cultivation applied.

In 1747, a small field of wheat was a great curiosity at Edinburgh, and, up to 1770, very little grew there; now, it is abundant It is, then, to superior light and heat combined that we are indebted for our great crops of cereals; it is to the same vivifying causes, in greater degrees, that the West Indies owe their pine-apples and highly flavored fruits which we in vain attempt to produce without artificial means.

It may be considered an axiom in horticulture, that all plants require the soil as well as the atmosphere in which they grow, to correspond in temperature with that of the countries of which they are natives. The mean temperature of the soil should be above that of the atmosphere; how much above, depends upon climate and season. The earth is wanner than the atmosphere, as a general rule. When plants are cultivated in glass houses, there is little difficulty in supplying them with the amount of bottom heat which they may require; but this can either not be effected at all, or only to a limited degree, by a selection of soils and situations, when plants are cultivated in the open air; and hence, one of the many difficulties of acclimatizing in a cold country the species of a warmer climate. It is true that plants will exist within wide limits of temperature, and, consequently, a few degrees of difference in the natural bottom heat to which they are exposed, may not affect them so far as to destroy them; but it cannot be doubted that the conditions most favorable to their growth, are those which embrace a temperature rather above than below that to which they are accustomed in their native haunts.

This point ascertained, we have come to an important discovery regarding acclimation.