GAS INTERCHANGE. 83 



shown in table 62. However, the results of the two controls carried on in the 

 dark show that the same general conditions prevail. 



Evolution of carbon dioxide by plants in diffuse light has been noted before 

 by Garreau and in cacti especially by Aubert, but in none of the case? cited 

 by these writers was there so considerable an amount of carbon dioxide formed. 

 It seems hardly possible that so large an evolution of this gas takes place at 

 all times of the year, for the actual loss of substance is not inconsiderable. 

 It must be remembered, however, that under natural conditions the cacti are 

 not exposed to a subdued light and the relatively feeble photosynthetic action 

 of the limited amount of chlorophyll-bearing tissue is not enough to outweigh 

 the action of light in breaking down the organic acids present. 



What is more surprising than the evolution of carbon dioxide in diffuse light 

 is the considerable amount that is given off when the plants are exposed to 

 the direct rays of the sun (table 63) . As has already been described, the manner 

 of exposing the cactus joints in a closed vessel to the sun's rays was to immerse 

 the container in ice-cooled water. It is realized that under such conditions 

 the full effect of the sunlight might not be obtained, but nevertheless, there 

 is a marked effect. The results corroborate those by the Pettenkofer method, 

 which have been given in detail. Unlike the action in diffuse light there is 

 no oxygen absorbed from the external air in normal active tissue either young 

 or old; instead, an increase in the oxygen content was observed. It may 

 reasonably be supposed that the photosynthetic processes afforded the oxygen 

 necessary for the breaking down of the acids, and that which escaped was the 

 surplus not used in these disintegration operations. The actual amount of 

 oxygen given off varied considerably ; in one case it was minimal, but in three 

 other instances it averaged 0.45 c.c. per gram hour. The evolution of carbon 

 dioxide, while much less than that found in diffuse light, was by no means 

 negligible, but here also there is a great variation in amounts. In one case it 

 was 0.072 per gram-hour, dry weight, in another as high as 0.432. It is notice- 

 able that in experiments TT and UU (table 63), where there is more carbon 

 dioxide, there is less oxygen, and vice versa. In the one instance in which young 

 material was used, there was a very large amount of carbon dioxide formed, 

 nearly 0.84 c.c. per gram-hour, dry weight. 



As would be anticipated, the acidity was falling rapidly in all these sunlight 

 experiments, and it is probable that the photochemical action of the sun's rays 

 caused such a rapid splitting of the organic acids that the amount of carbon 

 dioxide produced was more than could betaken care of by the chlorophyll tissue. 



In the two experiments tried with flaccid material, absorption of oxygen was 

 indicated even under exposure to sunlight. The gas ratio is rather more than 

 unity, and of about the same magnitude as that found in the dark (see table 

 56 for comparison). It seems questionable, then, if in the flaccid plants there 

 is more than a much reduced photosynthetic activity or perhaps none at all. 

 A mere external examination of an old, thoroughly flaccid joint suggests at once 

 that its photosynthetic activity is probably minimal, for the color is commonly 

 far from the normal green, and the chlorophyll appears not to be simply 

 masked, but really of a yellowish color. It is true that for the amount of 

 carbon dioxide produced, there is less external oxygen absorbed in these plants 



"Garreau. Annal. de sci. nat. Botanique, series 3, vol. xv, p. 6, 1851. 



