36 ACIDITY AND GAS INTERCHANGE IN CACTI. 



for experiment than for any other reason. No determinations at all were 

 made of the acidity of tissues more than 3 years old, though it is highly 

 probable that whatever differences they might show would be along the same 

 lines indicated in the 3-year-old shoots. 



The periodicity of the acid content of cacti other than Opuntia versicolor was 

 touched on in a number of experiments which are set forth in tables 23, 24, and 

 25. In table 23 are given the results of three series with Mammillaria 

 grahami over a period of 26 hours with determinations at intervals usually of 

 2 hours. The graph which is shown in figure 4 is from an average of these 

 three series and is essentially the same as that determined for Opuntia versi- 

 color. Of necessity each determination was made with a separate plant, since 

 the individuals of this species are not large. Hence the curve does not, as 

 in the experiments with Opuntia versicolor, represent the activity of single 

 individuals and for that reason may show irregularities due to the varia- 

 tion of different specimens. Nevertheless, the decline and rise of the acidity 

 is plainly indicated. So far as the extraction of the juice is concerned, Mam- 

 millaria grahami is perhaps better than Opuntia versicolor, as there is an entire 

 absence of glutinous matter. It is, however, not sufficiently common to 

 allow of its extensive use in experiments. 



A few determinations, made at three periods during the day, of Opuntia 

 leptocaulis, 0. discata, and O. blakeana, are shown in table 24. The decline 

 that is to be expected during the course of the day shows clearly. In all of 

 these, however, the amount of glutinous substance is very large and the 

 acidity determinations are more relative than absolute. However, there is 

 plainly less acid at 4 p. m. than at 9 a. m. The actual averages of the experi- 

 ments shown are as follows: At 9 a. m. three determinations of 0. leptocaulis 

 had the fresh- weight total acidity per gram of 1.85; at noon it had sunk to 

 1.07, and still later, at 4 p. m., it was only 0.83, a marked decline, though not 

 so great as that found in 0. versicolor. Similarly three sets of 0. discata gave 

 an acidity at 9 a. m. of 0.98; at 12 noon of 0.69, and finally at 4 p. m. of 0.35. 

 The single series with O. blakeana is similar in rise and fall of acidity. 



The series with Carnegiea gigantea are somewhat more fragmentary. Five 

 sets of determinations were made between April 5 and 26, 1911, in which, 

 however, both total and juice acidity were determined. At the 3 hours taken, 

 namely 9 a. m., 12 noon, and 4 p. m., the total acidity per gram was respectively 

 0.79, 0.59, and 0.43, while the juice acidity per cubic centimeter, taken simul- 

 taneously, was 0.91, 0.68, and 0.49. Thus here, as in the other cases, there 

 was the characteristic drop in acidity as the day advanced. 



All of these miscellaneous experiments corroborate the more elaborate results 

 with Opuntia versicolor, and unquestionably the same reactions would be ob- 

 served in these cases as were found in the latter species. 



