VEGETABLE.] PHYSIOLOGY 49 Ivients piaot- lements i ash. Nitrogen. Nitrogen is absorbed in the form of ammonia and its compounds and of nitrates ; it may also be absorbed in the form of organic nitrogenous compounds. The researches of Lawes, Gilbert, and Pugh, as also those of Boussingault, have proved that plants are incapable of assimilating free nitrogen. It appears that, on the whole, nitrogen absorbed in the form of ammonia compounds is more readily assimilated by plants than nitrogen absorbed in the form of nitrates. Pasteur lias shown, for instance, that the Yeast plant cannot assimilate nitrates. Sulphur. Sulphur is absorbed from the soil as sulphates, those of ammonium, potassium, magnesium, and calcium being the most advantageous. It may also be absorbed to some extent in the form of organic compounds. Phospftarus. Phosphorus is absorbed from the soil in the form of phosphates. Besides being a constituent of certain substances allied to the proteids, such as miclein and plastin, phosphorus seems to bear an important relation to certain of the metabolic processes. Phosphates are to be found especially in those parts of plants which are rich in protoplasmic cell-contents. It appears that a supply of phosphates promotes considerably the assimilation of nitrogen by the plant. Potassium. Potassium is absorbed in the form of a variety of salts, of which the chloride is the most advantageous form, according to Nobbe. Like phosphorus, it is to be found in largest quantity in those parts of plants which are rich in protoplasmic cell-contents. It appears to have an important influence on the constructive meta bolic processes of plants which contain chlorophyll. Nobbe found, in the case of a Buckwheat plant, that in the absence of a supply of potassium its growth was diminutive, and that the amount of starch in the plant was very small. On the addition of potassium chloride to the water -culture the starch -grains became more numerous in the chlorophyll-corpuscles, and made their appearance also in the tissues of the stem. The precise significance of potas sium in relation to these processes is not known. Liebig was of opinion that it played an important part in the distribution of carbohydrates throughout the plant, but this view has not been confirmed. It appears rather that the facts upon which this view was based point to an effect due not to the potassium itself but to the particular salt of it which was absorbed (see " chlorine " below). There can be no doubt, however, that potassium bears some im portant relation to the formation and storing up of carbohydrates, for it is always present in large quantity in organs, such as leaves, tubers, seeds, &c. , in which these processes especially take place. Calcium. -The compounds in which calcium is usually absorbed are the sulphate, phosphate, nitrate, and carbonate, the last-named salt undergoing decomposition in the process. It appears that the chloride is injurious to plants. The precise use of calcium is un known. It very commonly occurs in the cells of plants in the form of crystals of the carbonate or the oxalate, and possibly one of its important functions is to form insoluble salts with acids which are of no further use in the plant, and are even injurious to it. Magnesium. Like calcium, this may be advantageous!) absorbed in the form of all its salts, except the chloride. Nothing definite is known as to its use. Iron. It appears that iron may be absorbed in the form of any of its salts. It is known to be essential only to those plants which contain chlorophyll. If a seedling be cultivated by the method of water-culture, with its roots in a solution which contains no iron, the leaves formed will be successively paler in colour until at length theyare nearly white ; in this state the plant is said to be "chlorotic." If a small quantity of a salt of iron be then added to the solution in which the roots are, or if the pale leaves be painted over with a dilute solution of iron, they will soon become green. Iron, there fore, plays an important part in connexion with the formation of the green colouring- matter chlorophyll. It is still a debated question whether or not iron enters into the composition of the chlorophyll-molecule. Chlorine. Chlorine is absorbed from the soil in the form of chlorides. The evidence as to its significance in the nutrition of plants is conflicting. Nobbe, Leydhecker, Beyer, and more recently Farsky have observed that water-cultures of Buckwheat, Barley, and Oats do not nourish when grown in solutions containing no chlorides, and since the chlorophyll-corpuscles of the plants become crowded with starch -grains it was thought that chlorine had some import ance in connexion with the translocation of carbohydrates. Knop and Dvorzak have observed, on the other hand, that ilaize plants will grow well in solutions containing no chlorine, and further, that the accumulation of starch in the chlorophyll-corpuscles may be induced by various abnormal external conditions. Sodium. This element is never absent from the ash of plants, and in some cases, especially in maritime plants, it is present in considerable quantity. It might be inferred from its constant occurrence in the ash that sodium is of some importance as a food- material ; it was thought, in fact, that it might serve as a substitute for potassium, but this has not been found to be the case. Its con stant presence in the ash is due merely to its universal distribution in the soil. Silicon. Silicon is absorbed in the form of soluble silicates, and possibly as soluble silicic acid. The silicates are brought into solution to some extent by the carbon dioxide present in the soil, and also by the acid sap of the root-hairs. It is always present in the ash of plants, sometimes in large quantity ; in wheat-straw, for instance, it constitutes 67 50 per cent, of the ash (Wolff). It was thought that silicon must be essential to nutrition. Sachs found, however, that a Maize plant will grow well in a water-culture from which it can obtain no silicon. On the other hand, Wolff has ascer tained that in the case of Oats the number of perfect seeds formed is greater when the plant is abundantly supplied with silicon. Constructive Metabolism (Anabolisni). When a plant is adequately supplied with appropriate Meta- food- materials, the external conditions to which it is Holism, exposed being favourable, it increases in weight, owing to an accumulation of the substances which constitute its organized structure. But this gain in weight is only rela tive ; for side by side with the constructive processes by which the food is converted into the substance of the plant processes, that is, which have as their result the formation of relatively complex from relatively simple chemical compounds there are going on destructive pro cessesprocesses, that is, which have as their result the formation of relatively simple from relatively complex chemical compounds which are attended by a loss of weight. The gain in weight by the plant represents the difference between the activity of the constructive and of the destructive metabolic processes respectively. The end of constructive metabolism is the formation of protoplasm. Protoplasm is certainly a very complex substance, though its precise constitution is unknown, and the food-materials of plants are much simpler substances ; there must, there fore, be a considerable number of processes to be gone through before protoplasm can be produced from the food- materials. We will now study these processes, and, in the first instance, confine our attention to those which have been ascertained to take place in plants which possess chlorophyll. It has been already mentioned that a green plant absorbs Building carbon dioxide when it is exposed to light. Under these U P of circumstances it also increases in weight ; it does not p y increase in weight when kept in the dark, nor when it is kept in an atmosphere from which all carbon dioxide has been removed. The absorption of carbon dioxide is then an indication that the plant is performing certain con structive processes, that it is assimilating carbon. The absorption of carbon dioxide is accompanied by an evolu tion of oxygen gas, the volume of the latter exhaled being approximately equivalent to that of the carbon dioxide absorbed. This is an indication that the absorbed carbon dioxide is undergoing chemical change. It seems probable that the change is of the nature expressed by the following equation xC0. 2 + a-H a O =(CH 2 6) -f x0. 2 that is, that from carbon dioxide and water a substance allied to formic aldehyde, or a polymer of it, is formed, free oxygen being evolved. It may be stated generally, with some considerable probability, that the first step in the constructive metabolism of a plant containing chloro phyll is the formation of a non- nitrogenous organic compound. It is just this formation of non -nitrogenous organic substance from carbon dioxide and water that the plant which is destitute of chlorophyll is unable to per form ; and it is on account of this inability that the carbon of its food must be supplied to it in the form of organic compounds, as pointed out above. The further processes of constructive metabolism appear to be much the same in all plants, whether they contain chlorophyll or not. The next step is probably the formation of some relatively simple nitrogenous organic substances from the nitrogen of the food and the non -nitrogenous organic substance
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