The controlling influence of carbon dioxide. Part III.—The retarding effect of carbon dioxide on respiration

Abstract

In Parts I and II of these researches we have been led to the conclusion that the resting condition of the moist seed, so often occurring in nature, is primarily a phase of autonarcosis under the action of the carbon dioxide produced by the seed itself. It has been shown that retardation and suspension of normal activity in plant protoplasm is produced by carbon dioxide in conditions otherwise entirely favourable to growth and during a stage normally characterised by vigorous growth. Attention has already been called to the striking analogy existing between the dormancy of the non-growing moist seed under the influence of carbon dioxide, whether maturing on the parent plant or showing delayed germination in the soil, and the dormancy of the unfertilised ovum. In both cases apparently simple causes are found sufficient to produce a change in the cell conditions owing to which the cell or tissue passes from dormancy into active growth by cell division. In neither case are the factors conditioning dormancy on the one hand and growth by cell division on the other as yet clearly established. A knowledge of these factors, however, must be of great importance both in physiology and medicine. In studying the phenomenon of carbon dioxide inhibition, as exhibited by dormant seeds, we seem to be in the presence of that fundamental question in physiology, the question of the cause of growth. Our problem, then, in these researches is clearly to determine what physiological changes accompany the inhibition of growth under the influence of carbon dioxide. Two hypotheses may be put forward. The inhibitory action of carbon dioxide may be the indirect result of a carbon dioxide effect upon the physical state of the protoplasm, for example, a change in its colloidal structure, its water-holding capacity, its permeability. On the other hand it may be of the nature of a direct chemical action producing a change in some phase of metabolism. Work to determine how far effects of the first class actually exist is still in progress. In the present paper one striking effect of the second class will be dealt with, namely, the retarding effect of carbon dioxide upon respiration. That oxidation and respiration are necessary for growth by cell division in plants is known. With regard to the animal ovum Loeb finds that oxidations are a controlling factor in the stimulus to growth, whether by natural or artificial fertilisation. The breaking up of the dormant condition of the egg and the beginning of cell division is accompanied by an essential increase in respiration. Should this increase be prevented either by a small dose of potassium cyanide or by the absence of oxygen, fertilisation, whether artificial or natural, has no effect, and growth does not ensue.