extent of the surface, the plant acquires an increased power of absorbing nourishment from the air, which continues in action far beyond the time when its derivation of carbonic acid through the roots ceases.  Humus, as a source of carbonic acid in cultivated lands, is not only useful as a means of increasing the quantity of carbon—­an effect which in most cases may be very indifferent for agricultural purposes—­but the mass of the plant having increased rapidly in a short time, space is obtained for the assimilation of the elements of the soil necessary for the formation of new leaves and branches.

Water evaporates incessantly from the surface of the young plant; its quantity is in direct proportion to the temperature and the extent of the surface.  The numerous radical fibrillae replace, like so many pumps, the evaporated water; and so long as the soil is moist, or penetrated with water, the indispensable elements of the soil, dissolved in the water, are supplied to the plant.  The water absorbed by the plant evaporating in an aeriform state leaves the saline and other mineral constituents within it.  The relative proportion of these elements taken up by a plant, is greater, the more extensive the surface and more abundant the supply of water; where these are limited, the plant soon reaches its full growth, while if their supply is continued, a greater amount of elements necessary to enable it to appropriate atmospheric nourishment being obtained, its development proceeds much further.  The quantity, or mass of seed produced, will correspond to the quantity of mineral constituents present in the plant.  That plant, therefore, containing the most alkaline phosphates and earthy salts will produce more or a greater weight of seeds than another which, in an equal time has absorbed less of them.  We consequently observe, in a hot summer, when a further supply of mineral ingredients from the soil ceases through want of water, that the height and strength of plants, as well as the development of their seeds, are in direct proportion to its absorption of the elementary parts of the soil in the preceding epochs of its growth.

The fertility of the year depends in general upon the temperature, and the moisture or dryness of the spring, if all the conditions necessary to the assimilation of the atmospheric nourishment be secured to our cultivated plants.  The action of humus, then, as we have explained it above, is chiefly of value in gaining time.  In agriculture, this must ever be taken into account and in this respect humus is of importance in favouring the growth of vegetables, cabbages, &c.

But the cerealia, and plants grown for their roots, meet on our fields, in the remains of the preceding crop, with a quantity of decaying vegetable substances corresponding to their contents of mineral nutriment from the soil, and consequently with a quantity of carbonic acid adequate to their accelerated development in the spring.  A further supply of carbonic acid, therefore, would be quite useless, without a corresponding increase of mineral ingredients.