Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/61

Rh VEGETABLE.] PHYSIOLOGY complex substances. For instance, when the seeds of leguminous plants, such as the Pea or the Bean, germinate, the quantity of proteid substance diminishes, and the quantity of amides, notably asparagin, increases ; there can be no doubt that the latter are derived from the former. It is well known that similar changes take place in the pancreatic digestion of animals, that leucin and tyrosin are formed from proteids, and that this is effected by an unorganized ferment termed trypsin &quot; ; it is quite possible that a ferment of this kind may be present in plants. Finally, there is probably, in cer tain plants at least, a ferment which converts cellulose into sugar. For instance, the Date seed contains a quantity of non-nitrogenous reserve material stored up as cellulose in its very thick cell-walls ; on germination this undergoes absorption and is conveyed to the embryo ; it is extremely probable that the conversion of the in soluble cellulose into some soluble substance (doubtless sugar) is effected by the action of a ferment. The penetration of the absorb ent organs of parasites into the tissues of their hosts is probably effected by the action of a ferment of this kind which is excreted by the parasite. But there are still other chemical changes to be accounted for as the result of which substances relatively rich in oxygen are produced from others which are relatively poor in that element. Some of these are, so far as is known, processes of simple oxidation, which go on as readily out side the organism as within it ; for instance, chlorophyll is oxidized quite as readily in alcoholic solution as when it exists in the chlorophyll -corpuscles of a plant; these processes of simple oxidation may then be regarded as going on independently of the vital activity of the organism. But there are other and more complex oxidations which may be termed &quot;oxidative decompositions&quot;; these involve something more than mere oxidation, and appear to de pend upon the vital activity of the organism. The following instances may be given to illustrate the nature of these changes. Ethyl-alcohol becomes oxidized, under the influence of a Fungus known as the Mycodcrma Aceti, as follows Acetic acid. C 2 H 6 + 0, = C,H 4 2 + H S 0. Another similar Fungus, the Mycodcrma Vini, induces a more com plete oxidative decomposition of alcohol, carbon dioxide and water being the products of its action. Again, a substance termed &quot;pyrocatechin&quot; and various organic acids occur in plants, and there seems reason, from the researches of Hoppe-Seyler and of Carl Kraus, to believe that they are derived from carbohydrates in some such way as the following Glucose. Pyrocatechin. Succinic acid. 3C 6 Hj 2 6 + 40, = C 6 H fi O, + 3C 4 H 6 4 + 3H 2 0, it being understood that this suggests only one of the ways in which the vegetable acids are formed. There are yet other processes of decomposition which, like the oxidative decompositions, are effected under the influence of living protoplasm, but which, unlike them, do not depend upon the presence of oxygen ; on the contrary, these decompositions, which may be generally termed &quot;fer mentations,&quot; depend upon the absence of free oxygen, for their activity is the greater the more limited the supply of this element. A characteristic example of this kind is afforded by the decomposition of sugar into alcohol and carbon dioxide, which is effected by Yeast, and is known as the &quot;alcoholic fermentation.&quot; Its nature is indicated by the following equation Again, various forms of Bacteria effect decompositions of this kind. Of these the putrefaction of organic matter, the lactic and butyric fermentations, are examples. It must not be supposed, however, that the property of exciting fermentation is confined to the protoplasm of lowly plants such as Yeast and Bacteria. It has been found that various fermentations are set up when living plant-organs of any kind leaves, flowers, fruits, seeds are kept in an atmo sphere which contains no free oxygen. The characteristic accompaniment of the destructive metabolism of plants, as of all living organisms, is, under normal conditions, that interchange of gases between the plant and the atmosphere which is known as &quot;respiration,&quot; and which consists in the absorption of oxygen and the evolution of carbon dioxide. It may be stated generally that the continual absorption of free oxygen is essential to the existence of at least the more highly-organized plants, and that in the absence of a supply of free oxygen they die. Death under these circumstances is to be attributed to the arrest of those metabolic processes which are accompanied by an evolution of kinetic energy in the organism that is, of the destructively metabolic processes ; and of these by far the most important is the self -decom position of the protoplasm. It would appear that the absorption of oxygen is essential to the self-decomposition of the protoplasm-molecule. It is impossible to say any thing definite as to the mode in which oxygen affects this process. Pfliiger has, however, suggested that the absorbed oxygen enters into the protoplasm -molecule as &quot;intra molecular &quot; oxygen, that the molecule is thereby rendered unstable, and that it then readily undergoes decomposition. In contrast to the plants which continue to live only Anaero- when supplied with free oxygen (the aerobia, as Pasteur bi tic has termed them) stand the anaerobia those, namely, P lants - which thrive best in the absence of free oxygen, and to which, in certain cases, the access of free oxygen is fatal ; of the latter, certain Schizomycetes and Saccharomycetes may be taken as examples. It is remarkable that it is just the anaerobiotic plants which are most highly endowed with the property of exciting fermentation ; and this, taken in conjunction with the fact that the activity of fermenta tion stands in an inverse relation to the supply of free oxygen, indicates the existence of some sort of correlation between the normal respiratory and the fermentative pro cesses. Jt appears that in aerobiotic plants the normal processes of destructive metabolism, of which the absorption of oxygen and the evolution of carbon dioxide are the out ward expression, may be replaced for a longer or shorter time by those abnormal processes of which fermentation is the outward expression; in completely anaerobiotic plants the fermentative are the normal processes. It is difficult to explain the physiological significance of fermentation, and to determine the manner in which it contributes to the maintenance of the life of the organism. Pasteur has suggested that it is the expression of an effort of the organism to obtain oxygen from substances which contain it in combination. Another possible view is that the organism obtains, by the fermentative decomposition of the substances upon which it acts, the supply of energy which, in the case of an aerobiotic plant, is afforded by the normal decomposition of its own protoplasm-molecules. The products of destructive metabolism are extremely numerous and of very different chemical nature. They may be. roughly classified into two groups : ( 1 ) the waste-jwoducte, substances which cannot be used in the constructive meta bolism of the plant, and which may be excreted ; and (2) the plastic products, substances which can enter into the constructive metabolism. 1. Waste- Products. Among the waste -products the Waste- most constant are carbon dioxide and water, which are products, exhaled in respiration ; it may, in fact, be stated generally that all living plants and parts of plants exhale carbon dioxide and watery vapour at all times. There is, however, no constant relation between the volumes of carbon dioxide exhaled and of oxygen absorbed in respiration, and the processes of destructive metabolism, of which the respira tory interchange of gases is the external expression, are so complex that the relation, whatever it may be, between the volumes of these gases in any particular case cannot be accounted for. The degree of independence between these processes is well illustrated by the fact that the absorption of oxygen is relatively greater at low temperatures, and that the exhalation of carbon dioxide is relatively greater at high temperatures. This seems to indicate that at a