SUMMARY AND CONCLUSIONS. 105 



begun when the original malic acid was formed. It is hard to see why in the 

 succulents the respiration should depend so largely upon the action of sunlight 

 when such is certainly not to be regarded as the case in the vast majority of 

 plants. It may be urged that we have acids in the tissues of practically all 

 plants and that in the presence of light these acids behave in a fashion similar 

 to that in the succulents, so that such processes are not peculiar to the fleshy 

 forms. However, in the ordinary plant the amount of carbon dioxide pro- 

 duction is large in proportion to the amount of acids found in the leaves, so 

 that in any case the latter could not cut much of a figure in respiration. 



It is as if when malic acid is formed a certain, and in the succulents a very 

 large, portion of the available food material were side-tracked out of the line 

 of the usual katabolic processes. Their further oxidation not taking place 

 rapidly, they accumulate in the tissues during the night, only to be destroyed 

 during the day by the action of light. As has been said before, the malic or 

 other acids may reasonably be supposed to exist as such in the vacuole and 

 are not to be regarded as in any intimate connection with the active protein, 

 but rather to be removed from the immediate sphere of protoplasmic activity. 



It is true that some deacidification occurs in the absence of light or under 

 high temperatures when the initial acidity is high, and consequently it may 

 be maintained that malic acid is utilized as a source of energy. That this 

 utilization of the malic acid should occur under such circumstances does not 

 seem to influence the general question. It may well be that in the prolonged 

 absence of light, when the more readily available substances have been disposed 

 of, the malic acid is attacked. Or in the case of high temperature, practically 

 the same thing happens, but more quickly, as the increased heat causes a more 

 rapid consumption of the food material. However, the katabolic disinte- 

 gration of malic acid is ordinarily not considerable and consequently it collects 

 at night, only to be broken down by the light during the daytime, but not in 

 connection with the protoplasm itself. According to the writer's point of 

 view, it would be necessary for the malic acid to form some union with the 

 living protein, just as sugar probably does, before the energy released in its 

 breaking down could avail the plant. 



The question of the evolution of carbon dioxide while under the illumination 

 of even direct sunlight, despite the photosynthetic processes which are going on 

 under such conditions, is a matter of particular interest. In the cases examined 

 there seems to be a greater production of this gas under these conditions than 

 has previously been reported. In diffuse light the rate of carbon dioxide evolu- 

 tion is considerable and some absorption of oxygen is still going on. It is 

 apparent, then, that in such relatively weak illumination the photosynthetic 

 activity must be feeble. In direct sunlight there is still some formation of car- 

 bon dioxide, but it is much diminished. The oxygen, however, ceases to be 

 absorbed in the young and in the mature-active tissue or may even be evolved, 

 though in the mature-flaccid plants the intake of the gas continues in even full 

 sunlight. In the last case it may be asked if photosynthesis is not practically 

 suspended, as indeed is indicated by the condition of the chlorophyll, which is 

 usually yellowish rather than green in color. Under any circumstances, 

 however, there must be an actual loss of weight even in the sunlight. Such a 

 phenomenon could not persist if the plant is to maintain itself for any length 

 of time, let alone to grow. It may be that the actual accumulation of ener- 

 gized substance on which the cactus lives and grows takes place during the 



