52 ACIDITY AND GAS INTERCHANGE IN CACTI. 



the apparatus was run without any plants in the receiver for 3 hours and there 

 was absolutely no indication of any precipitate in the barium-hydroxide 

 absorption tubes. It may be safely affirmed that whatever carbon dioxide 

 was found came from the plants themselves and that these plants were entirely 

 normal and healthy. 



The sunlight to which the material was exposed was as direct and as un- 

 influenced by interposed screens as was possible with the apparatus at hand. 

 The method, previously referred to, of immersion in clear water cooled with 

 ice was resorted to in order to prevent undue rise in temperature. It is real- 

 ized that some of the intensity of the sunlight is lost in transmission through 

 the glass and the water and that the results do not give the entire possible 

 effect of exposure to the sun. For this latter purpose it would be necessary 

 to utilize quartz or special glass for the receiver and cool it by some device 

 which did not require the immersion in water. With such an apparatus one 

 might indeed obtain an even greater evolution of carbon dioxide. The figures 

 are under rather than over the possible ones. 



.35 



FIG. 5. Graph showing rate of evolution of carbon dioxide by Opuntia versicolor at different 

 temperatures. In milligrams of COj per gram hour. The data are from table 31. 



WOUND RESPIRATION. 



That the small injury resulting from the removal of joints from the cactus 

 plants does not produce any measurable wound respiration has already been 

 shown in the description of experimental methods and precautions, and further 

 reference is unnecessary; but that a definite wound respiration follows a 

 considerable injury was established (tables 36, 37) and is of some interest, 

 especially in connection with the acidity relations. The sudden temporary rise 

 in the amount of carbon dioxide, which has been found with other massive 

 tissues immediately after wounding, was not observed. This abrupt rise has 

 been shown to be due to gas held in the tissue which escapes because of the 

 mechanical effect of the injury. It it not to be supposed that it is absent in 

 the case of the cacti, but rather that, owing to the great quantity of water in 

 these succulent plants, it is probably held in solution. During the third hour 



"Richards, H. M. The Respiration of Wounded Plants, Annals of Botany, pp. 531-582, 

 vol. 10, 1896. 



