not appear to be concerned with digestion so directly as the others is
pectase, which forms vegetable jelly from pectic substances occurring
in the cell-wall.
The enzymes which act upon glucosides are many; the best known are emulsin and myrosin, which split up respectively amygdalin, the special glucoside of certain plants of the Rosaceae; and sinigrin, which has a wide distribution among those of the Cruciferae. Others of less frequent occurrence are erythrozym, rhamnase and gaultherase.
The proteolytic enzymes, or those which digest proteids, are usually divided into two groups, one which breaks down ordinary proteids into diffusible bodies, known as peptones, which are themselves proteid in character. Such an enzyme is the pepsin of the stomach of the higher animals. The other group attacks these peptones and breaks them down into the amino-acids of which we have spoken before. This group is represented by the erepsin of the pancreas and other organs. A third enzyme, the trypsin of the pancreas, possesses the power of both pepsin and erepsin. The relationships existing between these enzymes are still the subjects of experiment, and we cannot regard them as exhaustively examined. It is not quite certain whether a true pepsin exists in plants, but many trypsins have been discovered, and one form of erepsin, at least, is very widespread. Among the trypsins we have the papäin of the Papaw fruit (Carica Papaya), the bromelin of the Pine-apple, and the enzymes present in many germinating seeds, in the seedlings of several plants, and in other parts. Another enzyme, rennet, which in the animal body is proteolytic, is frequently met with in plants, but its function has not been ascertained.
The digestion of fat or oil has not been adequately investigated, but its decomposition in germinating seeds has been found to be due to an enzyme, which has been called lipase. It splits it into a fatty acid and glycerin, but seems to have no further action. The details of the further transformations have not yet been completely followed.
Oxidases.—Another class of enzymes has been discovered in both animals and plants, but they do not apparently take any part in digestion. They set up a process of oxidation in the substances which they attack, and have consequently been named oxidases. Very little is known about them.
In many cases the digestion of reserve food materials is effected by the direct action of the protoplasm, without the intervention of enzymes. This property of living substance can be proved in the case of the cells of the higher plants, but it is especially prominent in many of the more lowly organisms, such as the Bacteria. The processes of putrefaction may be alluded to as affording an instance of such a power in the vegetable organisms. At the same time it must be remembered that the secretion of enzymes by Bacteria is of widespread occurrence.
Supply and Distribution of Energy in Plants.—It is well known that one of the conditions of life is the maintenance of the process which is known as respiration. It is marked by the constant and continuous absorption of a certain quantity of oxygen and by the exhalation of a certain volume of carbon dioxide and water vapour. There is no direct connexion between the two, the oxygen is absorbed almost immediately by the protoplasm, and appears to enter into some kind of chemical union with it. The protoplasm is in a condition of instability and is continually breaking down to a certain extent, giving rise to various substances of different degrees of complexity, some of which are again built up by it into its own substances, and others, more simple in composition, are given off. Of these carbon dioxide and water are the most prominent. These respiratory processes are associated with the liberation of energy by the protoplasm, energy which it applies to various purposes. The assimilation of complex foods consequently may be regarded as supplying the protoplasm with a potential store of energy, as well as building up its substance. Whenever complex bodies are built up from simple ones we have an absorption of energy in some form and its conversion into potential energy; whenever decomposition of complex bodies into simpler ones takes place we have the liberation of some or all of the energy that was used in their construction.
Since about 1880 considerable attention has been directed to the question of the supply, distribution and expenditure of energy in the vegetable kingdom. This is an extremely important question, since the supply of energy to the animal world has been found to depend entirely upon the vegetable one. The supply of energy to the several protoplasts which make up the body of a plant is as necessary as is the transport to them of the food they need; indeed, the two things are inseparably connected. The source of energy which is the only one accessible to the ordinary plant as a whole is the radiant energy of the rays of the sun, and its absorption is mainly due to the properties of chlorophyll. This colouring matter, as shown by its absorption spectrum, picks out of the ordinary beam of light a large proportion of its red and blue rays, together with some of the green and yellow. This energy is obtained especially by the chloroplastids, and part of it is at once devoted to the construction of carbohydrate material, being thus turned from the kinetic to the potential condition. The other constructive processes, which are dependent partly upon the oxidation of the carbohydrates so formed, and therefore upon an expenditure of part of such energy, also mark the storage of energy in the potential form. Indeed, the construction of protoplasm itself indicates the same thing. Thus even in these constructive processes there occurs a constant passage of energy backwards and forwards from the kinetic to the potential condition and vice versa. The outcome of the whole round of changes, however, is the fixation of a certain part of the radiant energy absorbed by the chlorophyll. The rays of the visible spectrum do not supply all the energy which the plant obtains. It has been suggested by several botanists, with considerable plaus1b1l1ty, that the ultra-violet or chemical rays can be absorbed and utilized by the protoplasm without the intervention of any pigment such as chlorophyll. There is some evidence pointing to the existence of this power in the cells of the higher plants. Again, we have evidence of the power of plants to avail themselves of the heat rays. There is, no doubt, a direct interchange of heat between the plant and the air, which in many cases results in a gain of heat by the plant. Indeed, the tendency to absorb heat in this way, either from the air or directly from the sunlight, has already been pointed out as a danger which needs to be averted by transpiration.
There is probably but little transformation of one form of kinetic energy into another in the plant. It has been suggested that the red pigment Anthocyan, which is found very commonly in young developing shoots, petioles and midribs, effects a conversion of light rays into heating ones, so facilitating the metabolic processes of the plant. This is, however, rather a matter of speculation. The various electrical phenomena of plants also are obscure.
Certain plants possess another source of energy which is common to them and the animal world. This is the absorption of elaborated compounds from their environment, by whose decomposition the potential energy expended in their construction can be liberated. Such a source is commonly met with among the Fungi, the insectivorous plants, and such of the higher plants as have a saprophytic habit. This source is not, however, anything new, for the elaborated compounds so absorbed have been primarily constructed by other plants through the mechanism which has just been described.
The question of the distribution of this stored energy to the separate protoplasts of the plant can be seen to be the same problem as the distribution of the food. The material and the energy go together, the decomposition of the one in the cell setting free the other, which is used at once in the vital processes of the cell, being in fact largely employed in constructing protoplasm or storing various products. The actual liberation in any cell is only very gradual, and generally takes the form of heat. The metabolic changes in the cells, however, concern other decompositions side by side with those which involve the building up of protoplasm from the products of which it feeds. So long as food is supplied the living substance is the seat of transformations which are continually proceeding, being partially decomposed and again constructed, the new food being incorporated into it. The changes involve a continual liberation of energy, which in most cases is caused by the respiration of the protoplasm and the oxidation of the substances it contains. The need of the protoplasm for oxygen has already been spoken of: in its absence death soon supervenes, respiration being stopped. Respiration, indeed, is the expression of the liberation of the potential energy of the protoplasm itself. It is not certain how far substances in the protoplasm are directly oxidized without entering into the composition of the living substance, though this appears to take place. Even their oxidation, however, is effected by the protoplasm acting as an oxygen carrier.
The supply of oxygen to a plant is thus seen to be as directly connected with the utilization of the energy of a cell as is that of food concerned in its nutrition. If the access of oxygen to a protoplast is interfered with its normal respiration soon ceases, but frequently other changes supervene. The partial asphyxiation or suffocation stimulates the protoplasm to set up a new and perhaps supplementary series of decompositions, which result in the liberation of energy just as do those of the respiratory process. One of the constant features of respiration—the exhalation of carbon dioxide—can still be observed. This comes in almost all such cases from the decomposition of sugar, which is split up by the protoplasm into alcohol and carbon dioxide. Such decompositions are now generally spoken of as anaërobic respiration. The decomposition of the complex molecule of the sugar liberates a certain amount of energy, as can be seen from the study of the fermentation set up by yeast, which is a process of this kind, in that it is intensified by the absence of oxygen. The liberated energy takes the form of heat, which raises the temperature of the fermenting wort. It has been ascertained that in many cases this decomposition is effected by the secretion of an enzyme, which has been termed zymase. This body has been prepared from active yeast, and from fruits and other parts which have been kept for some time in the absence of oxygen. The protoplasm appears to be able also to bring about the change without secreting any enzyme.
Expenditure of Energy by Plants.—The energy of the plant is, as we have seen, derived originally from the kinetic radiant energy of the sun. In such cells as are capable of absorbing it, by virtue of their chlorophyll apparatus, the greater part of it is converted into the potential form, and by the transport from cell to cell of the compounds constructed every part of the plant is put into possession of the energy it needs. The store of energy thus accumulated and distributed has to subserve various purposes in the economy of the plant. A certain part of it is devoted to the maintenance of
