EVOLUTION OF CARBON DIOXIDE. 51 



In the three controls in darkness, irrespective of temperature, there was an 

 average carbon-dioxide production of 0.43 mg. per gram-hour, fresh weight. 

 The same plants, under similar temperatures in the diffuse light from a north 

 window of the laboratory, gave out about half that quantity of the gas 

 0.21 mg. per gram-hour and, exposed to the most direct sunlight possible in 

 an inclosed apparatus, produced an amount of carbon dioxide per gram-hour 

 that was slightly more than one-third of that given out in darkness (i. e., 

 0.147 mg. per gram-hour). In the two series with young material a similar 

 state of affairs obtained, except that while the rate in darkness and diffuse 

 light is considerably higher than in sunlight, it is not correspondingly so. 

 The amount of carbon dioxide given off per gram-hour is as follows: darkness, 

 0.96 mg.; diffuse light, 0.48 mg.; sunlight, 0.18 mg. The greater photo- 

 synthetic activity of the young tissue probably explains its difference in this 

 regard from the mature. 



It is debatable whether we have in this case true respiratory activity, for 

 it seems probable that the evolution of the carbon dioxide, or at least a portion 

 of it, is due to some cause other than the action of the living protoplasm 

 itself. It is also probable that the actual source of the gas is from the mere 

 breaking down of the acids present by the effect of sunlight, largely irrespective 

 of the true respiratory functions. That the latter may be the case is indi- 

 cated not only by what we know of the action of light on certain organic acids 

 in vitro, apropos of which Spoehr's recent work has already been referred to, 

 but also by the behavior of the plants at temperatures as high as 60 to 65 C. 

 As the previously described results show, there is a considerable evolution of 

 carbon dioxide under high-temperature conditions where the protoplasm can 

 not possibly be living. In other words, then, it is possible that there may be 

 apparent evidences of respiration when that function is not really concerned. 

 However that may be, it is interesting to note that in some way more carbon 

 dioxide may be produced by the cacti in direct sunlight than can be utilized 

 by the photosynthetic machinery of the plant. 



In these experiments every possible care was taken to insure that the evo- 

 lution of gas in sunlight and diffuse light was not due to any extraneous organ- 

 ism. In the first place, the joints used were inspected with more than usual 

 care, both before and after the experiments, to see if any insect grubs were 

 present in the tissues. In every case no indication of any animal parasite 

 was to be found. The joints used were exceptionally healthy in appearance. 

 Although the possibility of the presence of bacteria in quantity sufficient to 

 affect the respiration markedly was exceedingly improbable, these organisms 

 were guarded against. In the last four series the joints were washed in a dilute 

 solution of formalin and then rinsed in freshly boiled distilled water. The 

 fact that control tests as well as the first two of the series showed no essential 

 difference in the amount of carbon dioxide evolved showed not only that the 

 question of the presence of superficially growing bacteria need not be consid- 

 ered, but also that the method of treatment with formalin was harmless to 

 the material used. In addition to these precautions, toluol vapor was drawn 

 through the whole system of the apparatus with the air-current. It was 

 always the practice in the use of the Pettenkofer apparatus to test it from time 

 to time for possible accidental sources of carbon dioxide, and in these experi- 

 ments it was subjected to especially careful scrutiny. At the end of the series 



