As is well known the plant contains considerable quantities of inorganic salts of sodium, potassium, calcium, magnesium and iron and the only source of these is the soil from which nitrates, sul phates and phosphates are also obtained. The water in the soil is not pure but is a weak solution of inorganic salts known as the soil solution. The exact composition of the soil solution is doubt ful. It must vary much from soil to soil and from time to time in the same soil and it is a matter of great difficulty to separate it from the soil. Estimations give a concentration of total solids in the soil solution from a wet soil of 0.1 to 0.025 per cent or less; a manured soil gives a stronger soil solution than an unmanured one. It is from the soil solution that the ordinary land plant takes up the salts necessary for its growth ; unless the essential salts can be supplied in sufficient quantity growth will suffer. It must be added however that a sufficient supply of water and appropriate salts together with a sufficient aeration of the root system are not in themselves sufficient; in addition, the reaction of the soil must be suitable. The soil must be neither too acid nor too alkaline, or to put it more accurately, the concentration of hydrogen ions in the soil must be correct. Olsen (1923) in studying Swedish vegeta tion demonstrated a remarkable agreement between the concen tration of hydrogen ions in the soil and the distribution of vari ous plants. The presence of lime or chalk in the soil is of great importance in relation to soil reaction since lime neutralizes acidity. The addition of lime will often raise an infertile "sour" soil to a fertile level. The striking response of some plants to lime-rich or lime-poor soils may be related to the hydrogen ion concentration of the soil.
Plants which normally grow in the soil can be grown successfully in water cultures, i.e., in a solution of salts, or in quartz sand watered with such a solution. A solution of this kind may consist of water 1,000 grams, potassium nitrate I gm., calcium sulphate 0.5 gm., magnesium sulphate 0.5 gm., calcium phosphate and ferric phosphate each 0.25 gm. In this or similar solutions plants may be grown successfully for many generations. If any of the elements, nitrogen, sulphur, phosphorus, potassium, calcium, magnesium and iron are absent, the plant suffers, indicating that these elements are essential to the plant. The sodium and chlorine which plants absorb from the soil do not seem to be essential for plant growth. Moreover, small quantities of boron are absolutely necessary for the proper growth of such a plant as the broad bean. We know, in addition, most of the essen tial salts appear to be poisonous when given alone and that salts have an antagonistic action to one another, each neutralizing the toxicity of the other. The culture solution has thus to be a bal anced solution in which the various toxicities cancel out.
When the water relations of the cell were discussed it was pointed out that the plasma membrane of the cell was of a semi-permeable nature, in that it held back soluble substances in the cell sap while showing a permeability to water. The fact that plants contain inorganic salts which they have taken up from the soil shows that the cell is not completely impermeable to dissolved substances, as does the fact that such substances as sugar travel about the plant. It is found, as we should expect that when salts (such as potassium nitrate or sodium chloride), or even cane sugar, are used as plasmolysing agents that the substance enters sooner or later and plasmolysis dis appears. The entry of salts into the cell raises the whole question of the nature of the living plasma membrane and the forces which are concerned in bringing about the passage of substances in and out of the living cell. The subject, however, is as yet a very con
fused one and has recently been reviewed by Stiles. On simple physical principles a salt should go on entering and accumulating in a cell until the concentration inside and outside the cell is the same. If, however, a slice of carrot is placed in a solution of potassium chloride the salt goes on accumulating in the cell until the con centration inside is apparently many times (even 25 times) that outside. Again as Osterhout has shown, if the sap of the large cells of the seaweed Valonia is analysed it is found that the con centration of potassium inside the cell is over 4o times that of the sea water outside. On the other hand the concentration of sodium is five or six times as great in sea water as it is in the cell sap. The fact that the living plasma membrane can keep permanently be tween its two sides so great a difference of concentration as that of potassium shows that it must be of very peculiar nature.
As might be expected from the fact that the accumulation of substances in a cell is a fluctuating quantity one finds that the permeability of a cell is not constant; in other words the rate of entry or exit of a given substance may be changed by external conditions. For example the permeability of a cell to a particular dye may be markedly affected by the nature and concentration of a neutral salt in the dye solution. Since the plasma membrane contains a high percentage of protein and the physical condition of such substances is altered by many salts, the change in permeability is not unexpected. Then again we find that the rate of entry of one salt into a cell is altered by the presence of another salt. Weak solutions of sodium chloride and calcium chloride, which by themselves do not plasmolyse a cell, will do so when mixed. The effect seems to be due to the calcium ion altering the membrane in such a way as to hinder the entry of the sodium ion—another example of antagonistic action. Again it has been claimed that light increases the permeability of the cells in the leaf of the lime. Illumination has been definitely shown (Brooks, 1927) to increase the rate of entry of a dye into the Valonia cell, and also to increase the rate of diffusion of the salts from the cut surface of a tissue, such as that of the swelling at the base of the leaf-stalk in the sensitive plant (Mimosa). We have probably to view the permeability of active cells as con stantly varying, though how the changes in permeabilities of neighbouring cells are related so as to cause material to travel in any particular direction in a tissue is still obscure.
Transport of Salts and of Organic Substances.—The mechanism of the transport upward of mineral salts and the trans port downwards of organic substances (such as sugar from the leaves), is still imperfectly known. Owing to the apparent slow ness of diffusion of salts from cell to cell the supplies of salts required by the leaves w ould seem necessarily to move in the transpiration stream. There are, however, no data available showing that the rate of the stream and the concentration of salts in it are sufficient to transport the necessary supplies. A reduction of the rate of the transportation stream by shading leaves does not appear to reduce the salt content of the plant, at least in the herbaceous plants investigated. It may be that a certain critical rate of the stream is required and that in the experiments men tioned the rate was always above this.