12 ACIDITY AND GAS INTERCHANGE IN CACTI. 



they were present, care had to be taken not to make false reading of the end- 

 point because of the uneven coloration which ensued, owing to the lack of 

 homogeneity of the extract. It was found that by constant agitation the 

 mucilaginous substance could be made to color evenly with the more fluid 

 portions of the sample. 



The actual total acidity, of course, will be the multiple of the aliquot portion 

 titrated, plus whatever acid has been estimated as pure juice. In the earlier 

 experiments the acidity of the pure juice was not determined, as water was 

 used in connection with the first pressing, but this method was soon abandoned. 

 The acidity of the pure juice is the quickest means of obtaining the relative 

 acidity of a number of specimens, and, as it varies closely in accordance with 

 the total acidity, it will serve at times for the necessary index of the condition 

 of the tissue. In some of the later work, where speed was a necessary factor, 

 the total acidity was not determined. In all of the tables in which acidity is 

 recorded it is indicated which of these three ways was used, i. e., total acidity 

 alone, acidity of juice alone, or the two together. It will be noted that the 

 total acidity in most cases has been calculated both as to amount per gram 

 fresh weight and per gram dry weight. The importance of the latter becomes 

 evident when we wish to compare the acidities of flaccid specimens with those 

 of turgid specimens. The former showing, as they do, so much higher per- 

 centages of dry weight, the acidity per gram solid substance becomes even less 

 than it appears when reckoned according to fresh weight. 



In all cases the tables show the acidity in terms of cubic centimeters of 

 N/10 KOH, either per cubic centimeter of the pure juice of per gram fresh or 

 dry weight of the material used. For the sake of brevity it is commonly 

 stated that the acidity was such and such a figure per cubic centimeter or 

 per gram, meaning by this that that was its value in terms of cubic centimeters 

 of the decinormal alkali solution. 



Occasional exceptions to the praxis as outlined were made where demanded. 

 In a few cases, where only very small samples of the cactus joints were of 

 necessity available for acidity determination, it was not possible to obtain as 

 much as 10 c.c. of the pure juice, so a smaller amount had to suffice. The use of 

 a smaller quantity is of course less satisfactory and was not resorted to when it 

 was avoidable, but, inasmuch as the results obtained were not discordant with 

 those from larger samples, it is fair to assume that they were sufficiently 

 trustworthy. Another departure from the usual practice was the occasional 

 determination of the acidity in the whole of the residual pressings without 

 making the liquid up to a known volume from which an aliquot part could 

 serve. While this method is not necessarily inaccurate, it is attended with a 

 certain risk of losing the whole titration. It may be said that this was only 

 done when, as in the other case, there was only a small quantity to work with. 



In the work done in 1912 and subsequently, all of the solid matter from the 

 samples of tissue used was recovered as carefully as possible and was dried, to 

 be used later in determining the dry weight. At the time of making the acidity 

 determinations, it was found convenient merely to dry the material in the 

 sun a process which takes place very rapidly in Tucson and to leave the final 

 drying to be done at convenience in the winter. In weighing the samples of 

 the fresh material it was not usually the practice to carry them down closer 

 than 0.1 gram, but with the dried specimens the weight is given to 0.05 gram. 



