General Metabolism of the Plant

Anaerobic Respiration and Fermentation.

Green plants which normally show aerobic respiration are still able to give out oarbon dioxide when kept in an atmosphere free from oxygen ; at the same time alcohol is produced in small quantities indirectly showing kinship between fermentation and this type of respiration. The alcoholic respiration of yeast has been the subject of much investigation and is known to be associated with an enzyme zymase, and the production of a compound of sugar and phos phoric acid (known as hexosephosphate) and an enzyme which acts on this compound. (See FERMENTATION.) The work of Neu berg and his collaborators indicates that pyruvic acid is an inter mediate product between sugar and alcohol.

Mechanism of Respiration.

The peculiarity of respiration as a chemical process lies in the fact that the plant (and animal) is able to oxidize at quite ordinary temperatures such a substance as sugar, which in the laboratory can only be burnt at high tem peratures. The question arises as to the peculiar conditions in the plant responsible for this. Oxidizing enzymes have been claimed as the key to the problem, but while these will readily oxidize certain aromatic compounds they will not act on the re spiratory substances such as sugars. Wieland has brought forward an entirely different theory of physiological oxidation in which water is the real source of the oxygen used. In this view the first step in the process is the combination of the respiratory material with water, which is followed by a process of dehydrogenation, probably under the action of an enzyme, in which hydrogen is split off leaving the material oxidized. The hydrogen is in an active state and in ordinary aerobic respiration combines with the oxygen of the air to form water ; the oxygen of the air thus acts as a hydrogen acceptor as it is called. In the absence of air other substances, such as methylene blue or hydroquinone, can take its place. Thus the oxidation of acetic acid to alcohol can go on without oxygen if methylene blue is present. Glucose also can give carbon dioxide at ordinary temperatures when water and some catalyst, such as palladium black, are present and some ac ceptor of hydrogen is available. Another theory is that of 0. Warburg, who holds that respiration is essentially a surface reac tion. He has shown that various substances such as oxalic acid, amino acids, and even sugars are respired in the living organism. The effect of narcotics on the "respiration" of the surface is very similar to that on the respiration of the living organism. Quastel's theory of the activation of the respiratory substances and the discovery by Sir Frederick G. Hopkins of the important sulphur containing substance glutathione must be referred to. The various theories of oxidation are discussed in the English edition of Kostychev's book on plant respiration (see references).

The term growth is generally applied to the increase in size of an organism. In unicellular organisms it applies only to the increase of the single cell; in multicellular organisms it is gener ally associated with the formation of new cells as well as with the increase in size of cells already existing. In the growth of the cell we may find an increase in all the cell constituents, cell-wall, protoplasm, etc., though frequently an increase in volume is brought about by the enlargement of the volume of the cell with out any increase of protoplasm. Of the manner of growth of

protoplasm we know very little. Bacteria (which are unicellular and consist of little else than a mass of protoplasm) when placed under the more favourable conditions of temperature and food supply will multiply so rapidly that each cell divides into two new ones every 20 minutes. The protoplasm thus grows at such a rate that its mass becomes multiplied eight times every hour. This shows that the production of protoplasm from non-living mate rial may go on at an astonishing rate, but of the details of this production we are quite ignorant. In cells invested with a cell wall the growth of the latter is due to the activity of the proto plasm. The new wall which divides the parent-cell into two new ones is formed across the nuclear spindle. (See above, Cytol ogy.) There has been considerable dispute as to the method of thickening of a wall already formed, whether it is by the addition of new layers, i.e., apposition, or by the intercalation of new par ticles of cell wall material among the old—the so-called intus susception. It is probable that the first process is the more com mon but that both occur.

Effect of External Conditions on Growth.—It is found that on the whole light has a retarding effect on growth. When il luminated the actual length attained by the organ and the rate of growth are both less than in the dark. Dicotyledonous plants grown in the dark have generally much elongated stems and also small leaves but the actual amount of light required to prevent the etiolated condition is very small. Light is of course indirectly necessary for the growth of the green plant, as without it the leaves are unable to build up the food material. Temperature is the factor which has the most marked effect on the rate of growth, as is to be expected since the temperature so largely controls the rate of the processes on which growth depends. There is a mini mum temperature below which the growth of the plant or organ does not occur at all, a maximum above which the growth stops as a result of injury to the plant, and an optimum temperature, the one at which the rate is greatest. These, however, are not absolute terms for, as with all effects of temperature, the degree of temperature cannot be separated from its duration. This is so particularly in relation to higher temperatures which may be in jurious only when the &ration is prolonged. In the case of the roots of the cress plant, which have been closely investigated, the minimum is found about freezing point, o° C; the optimum, how ever, was found to vary with the duration of the exposure. If the roots were tested by means of an exposure of 31 hours to various temperatures, the optimum was at 3o° C; with an ex posure of 7 hr. at 29° C, while with an exposure of 14 hr. the optimum was pushed back to 27° C. The maximum also was found to fluctuate in a similar manner being higher with shorter times of exposure. Both light and temperature have also a formative effect on the plant in that these factors may affect the shape of plant organs; plants grown at high temperatures in weak lights are noticeably different in appearance from plants grown at a lower temperature and a higher light intensity. The formative effects of chemical substances is also often marked at least in the case of the lower plants. Another external condition necessary for the growth of most plants is a supply of oxygen.