VEGETABLE.] PHYSIOLOGY 45 if less active at first than that of A, would be continuous, and thus, over a relatively long period of time, the amount of it absorbed would come to be much greater than that of A. Sjeic As a matter of fact, it has been ascertained that when abs J- different salts or other substances are presented to the root Et of a plant they are absorbed in different quantities. And !> further, it has been ascertained that the different salts are absorbed in different proportions by the roots of different plants ; it is, in fact, upon this that the necessity for the "rotation of crops " depends. A striking illustration of this is afforded by a comparison of the amount of silica present in the ash of equal dry weights of gramineous and leguminous plants. According to Wolff 100 parts meadow-hay contain 27 01 per cent, of silica. ,, wheat-straw ,, 67 50 ,, ,, ,, red clover ,, 2 57 ,, ., ,, pea-straw ,, 6 83 ,, ,, The absorption of salts in certain proportions by a plant is the expression of what may be conveniently termed its "specific absorbent capacity." It must not be supposed that this term suggests that the roots possess any selective power by which they absorb this salt and reject that one, or by which this one is absorbed in larger proportion than that one. The question as to whether or not a particular sub stance will be absorbed is a purely physical one, dependent upon the relation between the molecules of the substance and the cell -wall and primordial utricle which they have to traverse, and in no degree dependent upon the useful ness or hurtfulnesa of the substance to the plant. The amount absorbed of any particular substance depends ulti mately upon the activity with which the plant chemically alters the substance after absorption. To return to the illustration just given. The great difference between the amounts of silica present in the ash of gramineous and of leguminous plants respectively is the expression of the fact that the former are capable of withdrawing relatively large quantities of absorbed silica from the sphere of osmotic activity, and depositing it in the insoluble form in the tissues, whereas the latter can only do so to a comparatively small extent. The specific absorbent capa city of a plant is simply a manifestation of its specific metabolic properties. L of The amount of the various salts absorbed is not, how- 1 >r P- ever, exclusively dependent upon the specific absorbent capacity of the plant, for it is materially affected by the composition of the soil. The larger the quantity of any substance presented to the roots, the greater, other things being equal, will be the amount of it absorbed. This does not mean that substances can be absorbed by the roots in solutions of any degree of concentration. It appears that the root-hairs can only absorb very dilute solutions ; but for the watery solution of any salt capable of being absorbed there is a certain degree of concentration at which the proportion of the amount of the salt absorbed to that of the water absorbed is the same as that of the solution. If the solution be more concentrated the propor tion of water absorbed will be greater, if the solution be more dilute the proportion of salt absorbed will be greater. This is the general " law of absorption " determined by the experiments of De Saussure and of Wolff. It must, however, be borne in mind that, though the proportion of salt absorbed is larger in the case of a dilute than of a more concentrated solution, yet the absolute quantity of it absorbed from a more concentrated solution in a given time is greater than that absorbed from a dilute solution. >sorp- 2. Absorption of Gases. An interchange of gases is con- n oi stantly taking place between the plant and the medium in which it lives in the case of terrestrial plants, between the plant and the air ; in the case of aquatic plants, between the plant and the water. When the plant is a simple one each of its cells is in direct relation with the external medium ; when it is of complex structure there is usually some means provided by which the more internal cells are brought into relation with it, namely, a continuous system of intercellular spaces which communicate with the exterior in terrestrial plants by certain apertures termed "stomata," in the epidermis of the leaves and young stems, and by others termed "lenticels," in the cortical tissue of older stems and of roots. The gases principally absorbed by plants are oxygen Oxygen and carbon dioxide. The former is absorbed by every and living cell, and at all times ; the latter is absorbed exclu- sively by cells which contain chlorophyll, and by them only when exposed to light. In the more highly -organized plants the cells which contain chlorophyll are confined almost entirely to the leaves, so that the leaves may be regarded as the organs by which these plants absorb carbon dioxide. It has been held that the stomata are of great importance in promoting the absorption of this gas by the leaves, but the experiments of Boussingault prove that this view is not well founded. He discovered, namely, that the upper surface of the leaves of various plants with which he experimented absorbed carbon dioxide more actively than the lower surface, although the upper surface had scarcely any stomata, whereas they were very numerous on the lower. The absorption of carbon dioxide by the leaves is directly effected by the superficial cells. Gases, like solid substances, are only absorbed in solu- Absorp tion by the cells of plants. They may be brought to the tion f surface of the cell- wall already dissolved in water, as in the case of submerged plants, or they may be dissolved from the atmosphere by the sap which saturates the cell- wall, as in the case of land -plants; in either case they reach the interior of the cell in solution. When a gas has been taken up at the surface it diffuses throughout the cell- sap ; and in the case of a gas like nitrogen, for instance, which is not chemically altered in the cell, the absorption of it will cease when the cell -sap has become saturated with it. If, however, the metabolism of the cell changes the chemical condition of a gas its absorption will be continuous. This accords with what has been said with regard to substances absorbed by the roots. Another analogy exists between the absorption of gases and the absorption of substances in solution, namely, that, just as the root can only absorb a solution below a certain degree of concentration, so the leaf can only absorb a gas below a certain degree of pressure. Let us take in illustration the case of carbon dioxide. The pressure of the carbon dioxide in the air is very slight (0 04 per cent, by volume). It was first observed by Percival that an increase in the quantity of carbon dioxide in the air is favourable to the nutrition of green plants ; De Saussure found that a considerable increase is prejudicial ; and subsequently Godlewski showed that the optimum proportion is from 8 to 10 per cent., that is, that carbon dioxide is most readily absorbed by the plant Avhen its pressure is about 200 times greater than in ordinary air. Boussingault found that when leaves are exposed to sunlight in an atmosphere of pure carbon dioxide at the ordinary pressure they cannot decompose it, but if the gas is at a low pressure (in his experiment 17 mm. of mercury) they can do so. Besides oxygen and carbon dioxide other gases are also Other absorbed by plants, but to a small extent only. Nitrogen S ases is absorbed in small quantities merely in virtue of its a solubility and diffusibility ; as mentioned above, it is not in any way acted upon by the cells after its absorption. It appears that ammonia may be absorbed from the air in the form of gas by the leaves, and that, when thus absorbed,
it contributes to the nutrition of the plant. Other gases,