All other circumstances being the same, the vigour and richness of vegetation are proportionate to the quantity of light and heat received. The functions of light and heat are different, but their combined and continued action is indispensable for the perfect development of vegetation. Light enables plants to decompose, change into living matter, and consolidate, the inorganic elements of carbonic acid gas, water, and ammonia, which are absorbed by the leaves and roots, from the atmosphere and the earth; the quantity of carbon consolidated being exactly in proportion to the intensity of the illumination, which accounts for the darker green tint of the tropical forests. Light acting in its chemical character is a deoxidizing principle, by which the numerous neutral compounds common to vegetables are formed. It is the principal agent in preparing the food of plants, and in all the combinations and decompositions the law of definite quantitative proportions is maintained. It is during these chemical changes that the specific heat of plants is slowly evolved, which, though generally feeble, is sometimes very sensible, especially when the flowers and fruit are forming, on account of the increase of chemical energy at that time. To the same cause, the phosphorescence of certain flowering plants and a few fungi, is supposed to be due. The action of heat is manifested through the whole course of vegetable life, but its manifestations take various forms suited to the period and circumstances of growth. Upon it depends the formation of protein and nitrogenous substances, which abound in the seeds, buds, the points of the roots, and all those organs of plants which are either in a state of activity, or are destined to future development. The heat received, acting throughout the entire organism of a plant, may augment its structure to an indefinite extent, and thus supply new instruments for the chemical agency of light, and the production of new organic compounds. The whole energy of vegetable life is manifested in this production, and, in effecting it, each organ is not only drawing materials, but power, from the universe around it. The organizing power of plants bears a relation of equivalence to the light and heat which act upon them. The same annual plant from germination to the maturation of its seed receives about the same amount of light and heat, whatever be the latitude, its rate of growth being in an inverse ratio to the amount it receives in any given time. For one of the same species, the more rapid the growth, the shorter the life.