Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/60

Rh PHYSIOLOGY [VEGETABLE. which has been either formed in the plant or absorbed as food from without. The nitrogenous substances thus formed are probably crystallizable bodies, such as asparagin and leucin, which all contain nitrogen in the form of the group NH.,. The derivation of these substances from the nitrogenous food when it contains nitrogen in the form of ammonia (NH 3 ) is sufficiently obvious. When, how ever, it consists of nitrates it appears probable that the nitrogen of the nitric acid has to be transformed into the nitrogen of ammonia, that is, to be combined directly with hydrogen ; it is probably owing to their inability to effect this transformation that some plants, as mentioned above, cannot be supplied with nitrogen in the form of nitrates. The first step in this transformation is probably, as Emmerling has pointed out, the decomposition of the absorbed nitrates by the organic acids, especially the oxalic, of the plant ; the liberated nitric acid then under goes chemical change, resulting in the formation of ammonia. It is impossible to say with precision how this is effected, but there can be little doubt that it does take place ; some direct evidence is afforded by Hosaeus s observation that ammonia salts were to be found on ana lysis in a number of plants which had been supplied with manure containing no ammonia. The next process is an increase in the size and complexity of the molecule, attended in certain cases by the introduction of new elements (S and P), the product being one of those sub stances which are known as &quot;proteids.&quot; The last stage is the formation of living protoplasm from the proteid and other organic substances. Forma The formation of nitrogenous organic substance may tion of take place in any living cell, and, unlike the formation of non-nitrogenous organic substance, it goes on quite inde- orgauic pendently of the presence of chlorophyll and of the action sxib- of light. But there is evidence to show that in green stance, plants it is especially in the cells which contain the chloro phyll that the process goes on. The experiments by which this evidence has been obtained were made on plants with distinctly differentiated leaves. Emmerling observed in the Bean that, whereas in the root a relatively large quantity of nitric acid could be detected, there Avas much less in the stem, and in the leaves none at all, and he inferred that as the nitrates are supplied to the leaves they are used up in the formation of organic nitrogenous substance. Further, from the researches of Kellner, Emmerling, Borodin, and others it appears that the leaves contain the above-mentioned crystallizable organic sub stances, asparagin, leucin, etc., in considerable quantity ; and it is quite possible that these substances may be formed synthetically in the leaves, though it is true that they may be formed in other ways as well. Finally, Pott has found that the proportion of proteid in the plant increases from the roots upwards towards the leaves, the proportion in the latter being about twice as great as that in the former of many of the plants which he analysed. The formation of living protoplasm from the organic substances elaborated from the food necessarily goes on in every living cell. It lias been already mentioned that de structive metabolism that is, processes of decomposition is active in living cells, and it is especially the protoplasm which is the seat of these processes. The maintenance of the life of the cell is therefore an indication of the fact that the activity of the destructive metabolism is at least equalled by the activity of the constructive metabolism. In a young cell the latter exceeds the former, so that the protoplasm is increased in quantity ; then for a time the two are approximately equivalent, until at length the destructive gradually gains the upper hand, and eventually the death of the cell is the result. Destructive Metabolism (Kotabolism). Just as all the processes by which increasingly complex organic substances are formed in the plant, and which intervene between the food-materials on the one hand and the protoplasm on the other, are designated collectively &quot; constructive metabolism,&quot; so all the processes of decom position by which relatively simple substances are produced from relative complex ones, and which intervene between the protoplasm on the one hand and the excreta and other waste -products on the other, are designated collectively &quot;destructive metabolism.&quot; Of all the various processes Self- of destructive metabolism the most fundamental is the dec0 - decomposition of the protoplasm. It appears that this sltioi! decomposition is spontaneous that it is, as PfHiger terms pi asm it, a &quot;self -decomposition&quot;; and it is, in fact, only so long as this self-decomposition is proceeding that protoplasm can be said to be living. The destructive metabolism of an organism is not, however, confined to the self-decom position of its protoplasm ; the various complex organic substances which the cells contain may undergo chemical change quite independently of their entering into the metabolism of the protoplasm. The most active agents Unor- in producing chemical changes of this kind are certain g&quot;&quot;ized bodies which are termed &quot; ferments,&quot; and are distinguished el as &quot; unorganized &quot; ferments from the so-called &quot; organized &quot; ferments, such as Yeast and Bacteria. But little is known as to their chemical composition, and nothing as to the peculiarity of chemical constitution upon which their char acteristic properties depend. The unorganized ferments which have hitherto been detected in plants may be classified, according to the nature of the chemical changes which they induce, in the following four groups. 1. Ferments which convert starch into sugar (diastatic ferments). These have been found to be very widely distributed in plants, and in fact it seems probable that a ferment of this kind is present in all living plant-cells. Their mode of action is generally indicated by the following equation Starch. Maltose. 2. Ferments which convert cane-sugar into glucose (inverting ferments). A ferment of this kind, termed &quot;invertin,&quot; has been obtained from Yeast ; it is probable that a similar ferment is present in succulent fruits, for they commonly contain a mixture of cane- sugar and glucose. The following equatio n will indicate the nature of the process Cane-sugar. Dextrose. La vulose. Ci,H 2. 2 O n + H,0 = C 6 H ia 6 + C 6 H j, 6. 3. Ferments which decompose glucosides. The most familiar members of this group are emulsin or synaptase, found in the Bitter Almond ; myrosin, in the seed of the Black Mustard ; crytli- rozym, in the root of the Madder. The following equation repre sents the decomposition of the glucoside amygdalin by emulsin Oil of Bitter Prussia rl Almonds. Acid. i = C-1LO + HCN + 2(C 6 Hio0 6 ). - zu n 11 - 4. Ferments which convert proteids that are indiffusible and may be insoluble in water into others (peptones) which are both soluble and diffusible. These, which are only active in the presence of free acid, are termed &quot; peptic &quot; ferments. They have been found in quantity in the latex of certain plants (Carica Papaya and Fic-us Carica) and in the liquid excretion of carnivorous plants. It is, of course, impossible to represent by an equation the nature of the chemical change which these ferments induce. It is probable that other ferments than these may be present in plants, but they have not yet been actually obtained. There is probably one which decomposes fats (glyeerides) into glycerin and the corresponding fatty acid, thus Olein. Oleic Acid. Glycerin. C 57 H 104 6 + 3H 2 = 3C 18 H M 2 + C 3 H 8 8. Miintz and Yon Kechenberg have pointed out that the quantity of free fatty acids in oily seeds increases very much during germina tion, and the only satisfactory explanation of this fact which can at present be offered is that it is the result of the decomposition of the fats, in the manner indicated above, by an unorganized ferment. Again, it was mentioned above that crystallizable nitrogenous organic substances, such as leucin, asparagin, and tyrosin, occur in plants, and it was pointed out that they may be formed synthetic ally. But there can be no doubt that they may be, and frequently are, formed analytically, that is, by the decomposition of more