The medium of their transmission into the vegetable organism is water, which is assisted in its solvent action by carbonic acid and ammonia. The same law of osmotic diffusion, which accumulates the gaseous food of the plant in the tissues of the leaves, keeps up a supply of food from the soil. Evaporation from the surface (foliage and stems) removes from the plant a portion of the water which the cells contain. Capillary action restores this waste of water, bringing up from the soil a fresh supply, which always contains mineral matters in solution. The vague idea of the older vegetable physiologists, according to which there is a constant circulation of sap in plants, an upward and a downward flow - the sap ascending in the outer wood to the leaves, there being elaborated, and returning through the inner bark to the roots, depositing new matter on its way - must be noticed here, as an exploded but still oft-repeated error. There is no evidence that there exists any but an upward and outward current - a current toward the vaporizing surfaces. The amount of ash and the proportion of its ingredients are different in different classes of plants, and in the various parts of the same plant.

As a general rule, the exterior or terminal parts of plants, as the bark, leaves, and chaff or fruit envelopes, give the most ash, 7 to 28 per cent.; while the wood of trees is poorest, yielding but 2/10 to 3 per cent. The same organ contains different quantities of mineral matters at different stages of its growth. Doubtless, part of the substances which we find in the ash of a mature plant have finished their active functions, and have been secreted as waste matters. Doubtless, too, a part of the ash is accidental, not necessary to or employed by the plant, but having entered the vegetable circulation merely from being dissolved in the water which the plant has absorbed. For these reasons there is often little agreement between the numerous analyses which have been executed on the ashes of the same species or even variety of plant, its composition being to a certain extent influenced by the kind of soil in which it grows. Yet there is a general uniformity of composition, and it is undoubtedly true that the organization of the elements, carbon, hydrogen, oxygen, and nitrogen, into the cell tissues, and their contents, requires the cooperation of the ingredients of the ash, and that the relation between them is quantitative and definite, though we may never be able to determine it accurately. - The atmosphere is invariable in composition, and furnishes supplies of oxygen. hydrogen, carbon, and nitrogen (water, carbonic acid, and ammonia), beyond what the natural vegetation of any country needs.

The soil is exceedingly variable in composition. When it can supply sufficient quantities of ash ingredients, it will produce most of the plants indigenous to its locality. It then is fertile. When there is a deficiency of ash ingredients in available form, or the absence of any one of them, the soil is barren. There is an important difference between natural, spontaneous growth, and artificial, forced production. Natural growth in general is slow. Cultivated growth is rapid. For the former, natural supplies are sufficient; for the latter, artificial supplies must be provided. For the former the supplies of atmospheric food are in excess compared to those of ash ingredients yielded by the soil (telluric food), so that in forests and prairies the former accumulate on the surface of the soil as dead foliage, which in its decay becomes a telluric source of atmospheric food. In the latter the reverse most usually occurs, so that the organic matter of the soil diminishes and must be renewed by manures.

To repeat, in artificial growth (intensive culture) the soil is made to perform not only its natural function of furnishing ash in-gredients, but also a part of the office naturally left to the atmosphere, viz.: the supply of carbonic acid and ammonia. - Soils consist of the more or less comminuted fragments of rocks, mixed with certain products of their chemical decomposition, and with some organic matter - debris of vegetation. The composition of the soil varies according to the rocks from which it originates. It is rare that large tracts of soil are exclusively derived from the rocks that now underlie them. Most of the soils of our northern and middle states are partly composed of materials transported from the far north during what geologists term the drift period. The soils of valleys are constantly enriched from the rocks of surrounding hills, so that the composition of soils is thus more uniform in a general sense than it otherwise could be. We constantly meet, however, with limited areas having soils of peculiar characters. We find beds of sand, gravel, clay, marl, and peat or muck. The mechanically coarser parts of soil, the gravel and sand, consist of the still unde-composed fragments of the rocks from which it has been formed.

A part of the finest (impalpable) portion of every productive soil is usually made up of clay, which is a product of the chemical decomposition of certain minerals, and which possesses properties of the highest moment in agriculture. Under the general name humus is comprehended the organic matter of the soil which has resulted from the partial decay of previous generations of plants. The mechanical texture and other physical characters of the soil have a controlling influence on its fertility. Unless the soil be permeable by the roots of plants, and preserve the proper degrees of warmth and moisture, vegetation cannot attain its maximum development, no matter how favorable may be its chemical composition. Assuming then that the soil is physically adapted for a cultivated vegetation, its fertility depends upon its furnishing the growing plant with continuous and abundant supplies of the different bodies that have been named as the elements of vegetable nutrition. The quantity of ash ingredients that the heaviest crop removes from a soil is small, compared with the whole weight of the soil taken to such a depth as is penetrated by the roots of plants.