ACIDITY DETERMINATIONS. 29 



conditioning factor, since the increase of light is coincident with the increase 

 in heat. The temperatures given in these graphs are those of the normal out- 

 door conditions. One especially interesting series is that shown in table 9, 

 where, in addition to the air temperature, there is given that of the interior of 

 the cactus joint. The latter was taken by inserting into the joint a thermom- 

 eter of very small diameter, which was allowed to rest before observations 

 were made, until any possible traumatic effect had become negligible. The 

 graph follows that of the air temperature in a general way, but differs in the 

 important fact that the range is greater. Its maximum is higher and its 

 minimum is lower than those of air temperature. In short, it crosses the graph 

 of the latter twice. This phenomenon concerning temperatures within cacti 

 was observed by Shreve a in the case of Carnegiea gigantea, and is perhaps 

 universal in these fleshy plants. What its significance may be is hard to say. 

 It is not difficult to understand why the maximum within the cactus joint 

 should rise above that of the outside air, because of heat absorption and pos- 

 sibly to a small extent increased metabolic activity, but it is not so easy to 

 explain why the minimum should fall below. It is perhaps possible that 

 this depression of temperature should be ascribed to transpiration, although 

 the activity of cacti in this respect is notably low. The reason could hardly 

 be found in endothermic chemical reactions. The accumulation of acid which 

 is taking place during this time is presumably due to oxidation processes, 

 which, while they may be only partial as compared with those that take 

 place in ordinary plants, are nevertheless exothermic in their nature. A closer 

 examination of this temperature relation would be of considerable interest. 

 As far as we are concerned at the moment, the point is to emphasize the differ- 

 ence between the acidity and the temperature curves. 



These results which have just been discussed had for their object the presen- 

 tation of the complete diurnal curve of acidity under the natural conditions 

 which surround the plant, the complete course of which has not been plotted 

 in a detailed fashion for any succulent plant. The oscillations in acidity are 

 known to be affected most importantly by light and temperature, and a 

 number of experiments were made in which these two factors were isolated as 

 far as possible, as is set forth below. 



EFFECT OF LIGHT ON ACIDITY. 



The method for exposing the joints to sunlight without the attendant diffi- 

 culty of overheating has already been described in the account of experimental 

 methods. It suffices to say here that the plants were kept at temperatures low 

 enough to obviate effects on acidity due to heat and yet exposed to direct sun- 

 light as it filtered through the glass receiver. The temperatures used varied 

 from 5 to 20 C., which, as subsequent discussion will show, is below the point 

 where heat markedly affects the amount of acid present. Under such condi- 

 tions there is a decided drop in acid-content after exposure for several hours 

 in the bright sun. The average acidity of the juice in all of the experiments 

 made in this manner (table 14) was in the beginning 1.40 per cubic centimeter, 

 but fell at the end of the exposure to 0.74 for the same amount. There was thus 

 a drop of practically 50 per cent. This is not, indeed, as great a diminution 



fl Shreve, F. The influence of low temperatures on the distribution of the giant cactus. Plant 

 World, vol. 14. p. 136, 1911. 



