STUDIES IN PLANT RESPIRATION AND PHOTOSYNTHESIS. 79 



and Escombe^ and of Puriewitsch ;- thus far no measurements have 

 been made which determine directly the amount of radiant energy 

 used in photosynthesis. Such data as we now possess have been 

 obtained from calculations based either on the amount of carbon 

 dioxid absorbed (Brown and Escombe) or on the heat of combustion 

 of the photosynthate (Puriewitsch). Fundamentally the measure- 

 ments of Brown and Escombe and of Puriewitsch are based upon the 

 same principle, i. e., taking the heat of combustion of the photosyn- 

 thate or of glucose, or its equivalent in CO2, as a measure of radiant 

 energy utilized. The notable facts in Brown and Escombe's studies 

 are that only a small proportion of the energy absorbed by the leaf 

 is used in photosynthesis and that the amount actually used is a 

 variable quantity. 



Puriewitsch's figures exhibit similar variations in the percentage 

 of radiant energy used in photosynthesis, ranging from 0.6 per cent 

 to 7.7 per cent. In these experiments the intensity of illumination 

 was high and normal air was used, so that the carbon dioxid was 

 probably the limiting factor. Under such conditions it is to be 

 expected that the percentage of radiant energy used in photosyn- 

 thesis would vary inversely with the intensity of illumination. This, 

 however, is not the case. He finds that the longer the period of 

 illumination the lower is the percentage of radiant energy utilized 

 in photosynthesis. Thus for Polygonum sacchalinense the time and 

 percentage of energy used are as follows: 1 hr. 20 min. 7.7 per cent; 

 2 hrs. 20 min. 3.7 per cent; 5 hrs. 1.1 per cent and 2.5 per cent. 

 Since the ''time factor" in photosynthesis does not become evident 

 under 25°, these variations can probably not be ascribed to it, and 

 we must assume that as the plant accumulates carbohydrates less 

 radiant energy is utilized. It can not be assumed, as Puriewitsch 

 does, that in his experiments this decrease is due to the accumulation 

 of the photosynthate, which would mean the interference by the 

 well-known Saposchnikoff effect — in brief, an inhibition of the 

 photosynthetic activity, probablj'- due to the accumulation of car- 

 bohydrates.3 This effect seems to become apparent in land plants 

 only after more prolonged exposure than in the experiments of 

 Puriewitsch. Unfortunately, Puriewitsch gives no data as to the 

 rate of respiration; but it is safe to assume, from the experiments 

 of Matthaei'' and our own on the relation of carbohydrate-content 



> Brown, H. T., and F. Escombe. Proc. Roy. Soc. London, B. 76, 29-111 (1905). 



2 Puriewitsch, K. Jahrb. f. wisa. BoL, 53, 210-254 (1914). 



» Saposchnikoff, W. Ber. d. deut. bot. Ges., 11, 391-393 (1893). 



EwART, A. J. Jour. Linnean Soc, 30, 439-443 (1896); 31, 573 (1897); Ann. of Bot., 11, 439- 

 480 (1897). 

 «Matthaei, G. Phil. Trans. Roy. Soc. London, B. 197, 50 (1904). 



Borodin, J. Mem. de I'acad. imp. des Sciences de St. Petersburg (Serie 7), 28, 4 (1881). 



