ACIDITY DETERMINATIONS. 



As will be seen, the bulk of determinations of acidity were made with 

 Opuntia versicolor for the reasons already given, but a few series were under- 

 taken with Mammillaria grahami, Carnegeia gigantea, Opuntia discata, and 

 0. blakeana. The first problem was to establish more definitely the course of 

 the well-known periodicity of acidity that is found in such succulent forms. 

 While many of the investigators who have worked on succulents have made 

 some researches into the behavior of cacti, the greatest number have been 

 interested in various crassulaceous forms, notably Sempervivum and Echeveria. 

 It seemed advisable, then, to obtain a complete record of the behavior of a 

 cactus type in this regard, to which end a number of series, embracing com- 

 plete 24-hour periods, were undertaken in order to plot the diurnal decrease 

 and nocturnal increase of the acid content of the tissues. 



PERIODICITY IN ACIDITY. 



Three complete sets, composed of two or more separate series, shown in 

 tables 8, 9, and 10, were carried through in March and April 1911, at Tucson. 

 The acidity in these experiments was taken at 2-hour intervals over somewhat 

 more than 24 hours. In addition, two partial sets, extending from early morn- 

 ing to late in the evening, over a period of 16 hours, as shown in tables 1 1 and 13, 

 were made late in June 1913. Of this pair the last (table 13) was with young 

 joints, the other with mature-turgid ones. The acidity was recorded hourly 

 except at the end, but for lack of time only the pure juice was determined. 

 All of these experiments tell the same story, namely, the marked diminution 

 of acidity during the daylight hours and the slow increase during the night 

 until the maximum is reached at about sunrise. These facts are in accord with 

 the general ones already well known. In addition, numerous other deter- 

 minations were made, all of which indicate a similar periodicity, which will 

 be discussed in their proper place. 



It will be seen that in general the differences between maximum and mini- 

 mum are greatest in the summer time, as, indeed, would be expected. When 

 the plants are supplied with water they are most active during that season, and 

 as the sunlight is stronger and the temperature higher, any effect of these 

 factors on the acidity would be more marked. In the three series of March 

 and April 1911 (tables 8, 9, and 10) the maximum acidity per gram fresh weight 

 is somewhat more than twice that of the minimum, the actual ratios of the 

 three series being 1.48 to 0.67, 0.76 to 0.31, and 0.70 to 0.31, respectively. 

 These may be compared with the results shown in table 13 for mature-turgid 

 joints, where the maximum acidity is to the minimum as 3 to 1 (actually in the 

 average of the three series, 29 to 10). It will be observed, also, that in these 

 experiments the acidity was taken as late as 10 h 30 m a. m., which was some 

 hours after the highest point had been reached. This increase in ratio is even 

 more prominent in the acidity of the pure juice, where in the spring the maxi- 

 mum is to the minimum as 3 to 1 (table 10), while in the summer the ratio is 

 more nearly 7 to 1 (tables 11 and 12). 



From these experiments it would also appear that in the summer there is a 

 greater difference between the total acidity per gram fresh weight and the 

 acidity of the pure juice per cubic centimeter than during the earlier months. 



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