Darwin has shown that the insectivorous plants, by means of their modified leaves, absorb complex compounds, and that these are of importance in their nutrition. Flies and other small insects may often be found clasped in the tentacles of the Drosera, and in those experiments small pieces of meat, when placed on the leaves, were dissolved after a time by the secretions of the leaf glands and absorbed. Hydrogen is absorbed by all plants in combination in the form of water or ammonia and its compounds, or in the complex substances mentioned above. Oxygen is taken up by plants, free or in combination in water or in salts. The free oxygen is especially concerned in destructive metabolic processes. The large quantities of this gas absorbed by plants, and especially by fungi, show conclusively its consumption in metabolic processes. The process known as the respiration of plants is the absorption of oxygen and the exhalation of carbon-dioxide. The researches of Garreau' show that two distinct processes are in operation when leaves are exposed to the light: in the one oxygen is absorbed and carbon-dioxide is exhaled; in the other, carbon-dioxide is absorbed and oxygen is exhaled. When the leaves are exposed to a very bright sunlight, carbon-dioxide is absorbed and oxygen is exhaled, and the activity of these processes is so much greater than the absorption of oxygen and the exhalation of carbon-dioxide, that it appears as if the former only were in operation. Gases, like solids, can be assimilated only in solution, and as they are soluble in water, the cell walls of submerged plants may absorb them, and the sap near the surface of land plants will dissolve the gases from the atmosphere. The sap of plants contains, in solution, carbon-dioxide, oxygen, and also a certain amount of free nitrogen. That this nitrogen does not enter into the metabolism of the plant seems completely decided by the experiments of Lawes, Gilbert, and Pugh; but the more recent experiments of Atwater and Hellriegel should be compared 3 1 Insectivorous Plants. 3 Phil. Trans., 1860. 2 Ann. d. Sci. Nat., ser. 3, t. xv. Amer. Chem. Jour., viii, Nos. 5 and 6. Zeit. d. Ver. f. d. Rübenzucker Industrie, Nov., 1886. in this connection, and the matter cannot be said to be definitely settled. I can only enumerate in this connection, without going into the subject, the possible sources1 of the nitrogen supply:— 1. Organic, nitrogenous matter. 2. The ammonia of the air, and of the ocean. 3. The nitrous and nitric compounds formed by combustion and by electric discharges. 4. Nitrogen fixed in the soil by microbes. 5. The free nitrogen of the atmosphere. 6. Mineral nitrates. The sap which is continually flowing through living plants is a watery fluid, holding in solution mineral matters, gases, and organic substances. The root hairs of plants penetrate the particles of soil and absorb the moisture from a film which surrounds each particle: this is known as hygroscopic water. In thallophytes, the absorption is effected by the cells of the thallus, and in epiphytes a membrane invests the air roots especially adapted for the purpose. The distribution of water takes place at least to some extent - by the same process as its absorption. It passes by osmosis from cell to cell, as it passes originally from without into the superficial cells of the plant. The direction of this movement is not necessarily constant. The proportion of water in each cell varies and the tendency to establish a fluid equilibrium will cause a current towards those tissues which are deficient. These statements apply equally to gases and other substances held in solution, which are needed for the continuance of the chemical and physical changes going on in the living cells of different parts of the plant. The changes are more active for different substances in different parts of the plant. The mineral substances absorbed by the roots pass up to the leaves, where they are concerned in the constructive metabolism going on in those organs. The products of these processes pass from the leaves to parts of the plant which are actively growing, and where plastic material is re 1 "The Economical Aspect of Agr. Chemistry." By H. W. Wiley. Proc. A. A. A. S., xxxv, 1886. quired, or to the seeds or other organs in which organic stores are being laid up. If the stems or plants are cut in the spring, a flow of sap proceeds from the cut surface of that portion of the stem which is connected with the roots. This fact was investigated by Hales.1 He concluded that there is "a considerable energy in the root to push up sap in the bleeding season." This force is termed the root pressure, and is the measure of the absorbent activity of the root hairs. The root pressure is not only manifested by causing the flow of sap; it also may cause the exudation of drops of sap on the surface. There is a marked periodicity in the flow of sap, which is not due to the immediate result of variations in external conditions, but is inherent in the absorbent cells themselves. The current travels from the roots to the leaves through the lignified cell walls of the wood of the plant. The activity of the exhalation of watery vapor from the plant is not the same from its surfaces. The refreshing effect of a shower on withered leaves is due to the moisture penetrating the soil and being absorbed by the root hairs. From experiments it has been shown that if the air is very moist, and the leaves dry, the leaf surfaces may absorb a little water. 2 The cuticle offers a certain amount of resistance to the passage through it of vapor; this is due to resinous or waxy substances contained in it. The Mexican ocotilla 3 offers a striking example. It grows in very dry and exposed parts of the country, where rainfalls are infrequent. The bark is chiefly composed of wax and resinous substances. Other substances, as well as water, can be absorbed by leaves, experiments having shown that if a drop of calcium sulphate solution be placed on a leaf, it will have disappeared in the course of a few hours. This is more rapid when placed on the under surface. Though it seems that leaves may absorb water and substances in solution under certain circumstances, 1 Statical Essays, 1, 1769 (4th edition). 2 Detmer and Boussingault. 3 H. C. De S. Abbott, Proc. 4. A. A. S., xxxiii. See p. 117. Boussingault, Ann. Chem. et Phys., sér. V, xiii; also Agronomie, VI, 1878. Mayer, Landwirthschaftl. Versuchs-Stat., xvii, 1874. the especial absorptive function of leaves is the absorption of gases, as has been already explained. The subject of the ash-constituents of plants is a very important one in this connection. The essential mineral constituents of plants have already been mentioned; silicon, fluorine, manganese, sodium, lithium, rubidium, cæsium, barium, aluminium, zinc, copper, titanium, iodine, and bromine have also been found among the ash ingredients of certain plants. The method of absorption of soluble mineral salts has already been described. A solution of insoluble salts is brought about in a different way. A soil rich in organic matter is always charged with carbon-dioxide, and this gas is also given off by the roots of living plants. Water containing this gas is able to dissolve calcium carbonate and some silicates that are insoluble in pure water. The presence of certain soluble salts in the soil brings about a decomposition and renders the insoluble salts more readily soluble. Finally, the insoluble salts are brought into solution by means of the acid sap which saturates the cell wall of the root hair. This acid is not carbonic acid, for its reddening of litmus paper is permanent. It has been shown by experiment that the chemical elements are not universally absorbed by roots in their combinations in the soil. The wide differences in the composition of the ashes of plants show that each plant is endowed with a specific absorbent capacity. It is upon this fact that the "rotation of crops" in farming depends. A gramineous plant1 is able to withdraw relatively larger quantities of silica from the soil than a leguminous plant. The latter can only do so to a very slight extent. The absorbent capacities of nearly allied species are very different; again, individuals of the same species yield different ash compositions, depending upon their vigor; and the absorbent capacity of the plant varies at different periods of its life. It has been stated that "similar kinds of plants, and especially the same parts of similar plants, exhibit a close general agreement in the composition of their ashes, while plants which are unlike in their botanical characters are also unlike in the proportions 1 Wolff, Aschenanalysen, 1871. 1 of their fixed ingredients." If an ash-constituent can pass through a cell wall, its absorption will take place independently of its use or harmfulness to the plant, but the absorption of essential inorganic constituents will depend upon its relation to the metabolism of the plant. The ash-constituents of a plant increase from the roots upwards to the leaves, a fact showing that the leaves are the organs in which more especially active chemical changes take place. The ash ingredients are usually present in each plant cell; in the cell wall, imbedded in the cellulose, and partly in the contents of the cell. The salts of the alkaline metals and of the sulphates and the chlorides of magnesium and calcium occur in the solution of the sap. Silica and phosphates of calcium and magnesium are mostly insoluble and exist in the tissues of the plant. 3 Water-culture experiments have shown the essential ashingredients. Potassium, like phosphorus, is always found in relation with living protoplasm. If the plant was not supplied with potassium, it grew very little, and very little starch was formed in the chlorophyll corpuscles of the leaves. On the addition of potassium chloride, the starch grains became more numerous in the leaves, and made their appearance in other parts of the plants. Potassium, doubtless, plays an important rôle in the formation and the storing up of carbohydrates, for the organs in which these processes are active, as the leaves, seeds, and tubers, are found to be the richest in this element. It has been observed that cæsium and rubidium can replace potassium in the food of certain fungi (mould, yeast, and bacteria). 5 Salm-Horstmar describes some experiments, from which he infers that minute traces of lithium and fluorine are indispensable to the fruiting of barley. The same investigator has concluded that a trace of titanic acid is a necessary ingredient of plants. 1 How Crops Grow, by S. W. Johnson, London, p. 145. |