Page:The American Cyclopædia (1879) Volume I.djvu/218

 194 AGRICULTURAL CHEMISTRY phere to the extent of ^-^ of the volume of the latter. It is rapidly absorbed by the leaves of growing plants under the influence of sun- light, and undergoes decomposition in the vege- table cells, carbon being retained and assimi- lated, while the oxygen is set free wholly or in part, and exhales from the leaves. Water, which always exists in the atmosphere in the state of vapor, is an abundant source of both oxygen and hydrogen. Ammonia, a compound of hydrogen and nitrogen, is the chief source of nitrogen to the plant. It is ever present in the atmosphere in the form of carbonate, though in exceedingly small quantity. Nitric acid, which is formed by the oxidation of am- monia, is also a source of nitrogen. The plant being fixed and at rest, its food must necessa- rily be in perpetual motion around the organs destined to take it up. The atmospheric food is kept in motion, not only by the winds, but more effectually by the osmotic force (exosmose and endosmose). When two or more gases of unequal density are brought in contact in a confined space, they will gradually diffuse into each other, until they form a homogeneous mixture. If into a mixture of gases any solid or liquid body be introduced, which can com- bine with and remove one of the gases, it first takes up those particles of this gas which are in its immediate vicinity; but as fast as the uniformity of the mixture is thus disturbed, the absorbable gas diffuses into the space which has become void of it ; and as new portions are removed, other new ones are presented, until the whole is absorbed. All the forms of plant food are soluble in water. In virtue of these physical laws, it is plain that the tissues of a growing plant must be constantly surrounded with water, and with carbonic acid and ammo- nia dissolved in this water ; and as these are removed by the assimilating processes of the vegetable, they are restored by osmotic diffu- sion, so long as the atmospheric supply suffices. The ash of agricultural plants consists of the phosphates, sulphates, silicates, and carbonates of potash, soda, lime, and magnesia, with small quantities of oxide of iron and manganese, and alkaline chlorides. Other bodies, as alumina, copper, and zinc, are found in some kinds of land plants. The living plant contains sulphur (and perhaps phosphorus) in a state of organic combination, in the various nitrogenous princi- ples, or in sulphurized oils. On burning these compounds, sulphuric and phosphoric acid re- sult. Portions of the potash, soda, lime, and magnesia are combined with vegetable acids (oxalic, tartaric, malic) in the living plants, but these compounds are converted into carbonates by burning. Silica exists probably in the un- combined state in many cases, as in the bam- boo (taba%heer stalks of grasses, and scouring rush ; but in burning it combines with potash, lime, &c., so that it is found as a silicate in the ash. That these ingredients of the ash are in- dispensable to the development of vegetation, is proved not only by their invariable occur- rence in normally developed plants, but by direct experiment. The cereal grains, for ex- ample, will not mature in a soil which is defi- cient in any one of the following substances, viz. : potash, soda, lime, magnesia, oxide of iron, oxide of manganese, silica, sulphuric acid, phosphoric acid, chlorine. These kinds of plant food are all derived from the soil, and enter the plant through its roots. The medium of their.transmission into the vegetable organ- ism is water, which is assisted in its solvent ac- tion 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 re- stores this waste of water, bringing up from the soil a fresh supply, which always contains min- eral 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 to- ward 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 -j^ to 8 per cent. The same organ contains different quantities of mineral matters at differ- ent stages of its growth. Doubtless, part of the substances which we find in the ash of a mature plant have finished their active func- tions, 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 composi- tion, and it is undoubtedly true that the or- ganization 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, car-