50 ACIDITY AND GAS INTERCHANGE IN CACTI. 



tures like 60 or 65 C., and it is doubtful if they are at 55 C. The carbon 

 dioxide given off at these temperatures is probably either that which is held 

 dissolved in the cell-contents or that which is formed by the action of the high 

 temperatures in breaking up unstable organic compounds, entirely aside from 

 life processes, or perhaps both. Indeed, in dealing with such massive tissues, 

 results which seem to indicate a real respiratory curve must be accepted 

 with great caution. That there does appear here to be a definite maximum at 

 45 C., as indicated by the diminished activity at 50 C., where the tissues 

 must still be alive, is an interesting point and merits further investigation, 

 bearing in mind, however, the possible sources of confusion mentioned above. 



That rise in temperature by itself affects acidity as well as the rate of respira- 

 tion has been shown, and this is further borne out by the behavior of the plant 

 under conditions necessitating intramolecular respiration. Data for the 

 production of carbon dioxide in air in comparison with that in hydrogen and 

 in nitrogen are given in tables 32, 33, and 34. At relatively low temperatures, 

 that is 20 to 21 C., the intramolecular respiration is less than two-thirds that 

 of the normal, the actual ratio from averages of the experiments carried out 

 being 28 to 17; but it is noticeable that at a higher temperature, to wit, 35C., 

 there is a much closer approximation of the two. Under these conditions the 

 ratio becomes more nearly 6 to 5, or, in other words, intramolecular respiration 

 increases more rapidly with rising temperature than does the normal. Why 

 these cacti have such a high intramolecular respiration is puzzling. That the 

 evolution of carbon dioxide can be due (directly, at least) to the diminution of 

 acid in the tissues does not seem probable, for their breaking down is to a 

 large extent dependent upon an adequate supply of oxygen. Rather, it serves 

 as an additional reason for not considering deacidification as the only source 

 for the evolution of carbon dioxide by the plant. If, as for good reasons 

 is usually assumed, the massive tissues of such succulent plants are not easily 

 accessible to the oxygen of the air, one can see, in this phenomenon of intra- 

 molecular respiration as in acid formation, a correlation of function with 

 morphological structure. While, of course, the carbon dioxide formed during 

 respiration diffuses out from the tissues into the surrounding air, it would seem 

 probable that, at night at least, the gases held in the tissues must be very 

 poor in oxygen, and consequently the conditions approach those where intra- 

 molecular respiration comes into play. The point is one of considerable inter- 

 est and demands further investigation, particularly in regard to the nature of 

 the inclosed gases and the role of intramolecular respiration in these plants. 

 It was interesting to note that the tissues in the core of those joints kept free 

 from oxygen and at a relatively high temperature for some time, were fre- 

 quently in a state of semi-liquefaction. 



The deacidification processes are known to be associated with the evolution 

 of carbon dioxide; and, since sunlight is one of the most active agents in 

 breaking down the plant acids, it seemed well to make a number of experi- 

 ments to discover if an excess greater than the photosynthetic processes could 

 utilize might be formed under illumination. Even under these conditions a 

 considerable evolution of carbon dioxide was found, a circumstance already 

 noted by Aubert a in the case of certain cacti and by Garreau b with plants under 

 feeble light. The results of these experiments are set forth in table 35. 



"Aubert. Recherches sur les plantes grasses. 2d part, p. 77, Thesis, Paris, 1892. 

 6 Garreau. Annal. de sci. nat. Botanique, Series 3, vol. xv, p. 6, 1851, 1892. 



