General Metabolism of the Plant

Cosmic Relations of the Photosynthetic Process.-As

al ready stated this process is the fundamental chemical process on the life of all animals and plants depend. A study of the land plant's relation to the sun shows it as a machine of compara tively low efficiency, since only a small part of the energy is stored by the plant, crops such as cereals and potatoes retaining only 2 to 35- of the energy received. This uneconomical working is in part due to the fact that so much of the sun's energy consists of invisible heating rays which the plant cannot use for food production. Furthermore the plant is somewhat more efficient than first appears as part of the material manufactured during the day is burnt up again in respiration and so lost. The proportion so lost is very variable, depending largely on the temperature at night; it may be taken as 20-5c) per cent. Another cause of the low efficiency of the plant, as has already been pointed out, is the low concentration of the raw material (the carbon dioxide) avail able for the process. It has been suggested that in early Car boniferous times when coal was formed, the concentration of this gas was higher. However this may be it is clear that the energy available from our rapidly lessening stores of coal and oil is really the energy of sunlight received by the earth many thousands of years ago.

Respiration and fermentation are closely allied processes. Both consist in the breaking down, generally by an oxidation, of com plex organic compounds, commonly sugar, into simpler ones with a liberation of energy. In respiration oxygen is usually necessary and the breakdown is complete, carbon dioxide and water being formed. In the second oxygen is not necessary and the chemical degradation is not complete, some organic substance such as alco hol being the end product. As is to be expected the release of energy for a given amount of sugar consumed is much greater in respiration than in fermentation, the first being a much more economical process than the other. By means of these two proc esses the plant gains supplies of available energy from the food material consumed. Such supplies of energy are necessary to maintain the dynamic equilibrium of the cell and to carry on various chemical processes in which energy is absorbed. In the higher plants some of the energy is also used in mechanical work.

Aerobic Respiration.-As

has already been stated absorption of oxygen and the production of carbon dioxide are characteristic of the majority of plants as they are of animals. The carbon dioxide is not the only product for if sugar is oxidized an equal number of molecules of carbon dioxide are formed together with a certain amount of heat. The term aerobic respiration is applied to this process in contrast with the one in which oxygen is not necessary. which is termed anaerobic respiration and is akin to fer

mentation. Various substances appear capable of being oxidized in respiration. sugars. fats, proteins. sulphur, ammonia. nitrites. methane, hydrogen, etc., but in the higher plants sugar seems to be the substance most commonly consumed. The action is an oxidation and may be expressed thus : together with a release of 674.000 calories of energy.

The intensity of respiration varies enormously in different plants and different organs of the same plant. Actively growing tissues such as meristematic tissues show the most intense respira tion, in some cases as in the developing spadix of arum (see ARA cEAE) the rate is as high as in warm blooded animals and the heat generated is sufficient to raise perceptibly the temperature of the inflorescence. Developing seeds also show active respiration: poppy seeds at i6° C give out in 24 hr. 122 cc. per gram of dry weight. Leaves pressed together in a receptacle where heat losses are small may in the process of respiration raise the temperature so much as to bring about their death. When sugar is used in respiration the amount of carbon dioxide given out is equal to the amount of oxygen taken in : the respiratory quotient is then one. In other cases it is widely divergent from unity espe cially when fats or proteins are being consumed.

Effect of Conditions on Respiration.

The rate of this proc ess is markedly affected by various external and internal condi tions. Temperature is one of the most active as is to be expected, since a rise of temperature tends to accelerate chemical processes generally. The effect on respiration is very similar to that on assimilation (see above) and again markedly depends on the duration of exposure to higher temperatures. The respiration rate is very low round about the freezing point, but rises with increase of temperature, being very approximately doubled for a rise of temperature of o° C. After a time, however, an injurious effect with rise of temperature sets in—no doubt an effect on the protoplasmic mechanism, since high temperatures affect colloids.

The rate of respiration is also affected by the amount of the available material. Increase in the concentration of sugar supplied tends to raise the rate, as can be shown by adding or subtracting sugar from the food supply of a fungus in culture. A reduction of the amount of oxygen in the air has not much effect on respiration until the amount is considerably reduced. On the other hand the accumulation of carbon dioxide—a product of respira tion—has a definitely depressant effect on the rate of both aerobic and anaerobic respiration.