STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 35 



EXPERIMENTAL RESULTS. 



1. The Normal Course of Respiration. 



It has been known for a long time that as the supply of carbohy- 

 drates in a plant diminishes the rate of carbon-dioxid emission also 

 decreases. This in general may be conceived of as a matter of mass 

 action, in that as the concentration of the available material to be 

 oxidized becomes less the rate of the reaction is decreased. It has 

 also been recognized, however, that these relations are not as direct 

 and as simple as was supposed at first, for there are upper and lower 

 limits where other complications or limiting factors enter and where 

 the general complex of reactions takes other courses and makes use 

 of different material.^ But in order to form some conception of the 

 behavior of chlorophyllous leaves apart from the photosynthetic 

 process it is necessary to know what might be termed the normal 

 course of respiration, meaning thereby the rate at which the stored 

 material is converted into carbon dioxid and water with the liberation 

 of energy. In many investigations on the photosynthetic process 

 the assumption that respiration proceeds at the same rate in the 

 light as in the dark has been accepted as definitely established, or 

 the possibility of any variation of this factor has been totally neg- 

 lected. In a practical sense we are interested in photosynthesis 

 as a process in which the amount of energy stored exceeds that 

 liberated by the organism. Nevertheless, if we are to formulate 

 any sort of conception of the synthetic processes or make a quanti- 

 tative measure thereof, the reverse action, going on simultaneously, 

 can not be neglected. 



No conception such as the basal metabolism in animals has been 

 introduced into plant physiology, and, on account of fundamental 

 differences in the two organisms, it is improbable that such relations 

 can be worked out. But the normal course of respiration, although 

 of necessity a rather arbitrary value, offers a base-line, as it were, 

 on which subsequent respiratory activity can in a sense be super- 

 imposed. Another feature of such respiration rates is that with 

 sufficiently short periods and accuracy of CO2 determinations it 

 becomes evident that these rates show irregularities which emanate 

 from the internal w^orkings of the leaves and are independent of any 

 external conditions. We shall call attention to one such irregularity 

 in the form of a sudden rise in the respiration rate after the leaves 

 had been in the dark for about 40 hours. 



In order to give a comparison of excised leaves with the normal 

 plant, the rate of respiration of an entire plant was determined. 

 As the analytical data in table 13 indicate, the plant has in the stems 



> Spoehh, H. a. The carbohydrate economy of cacti. Carnegie Inst. Wash. Pub. No. 287 

 (1919). 



