ACIDITY DETERMINATIONS. 27 



ties shows in the averages of the determinations given in table 12. The ratios 

 of the dry- weight acidity to the concentration of the juice are as follows: for 

 the young joints, high acidities 4.2 to 1, low acidities 3.3. to 1; for the mature 

 joints, high acidities 3.4 to 1, low acidities 2.3 to 1. 



The obvious explanation of this phenomenon would seem to be that with 

 high-acidity content there is an increased amount of water present in the 

 tissues, which would of course affect the concentration of the juice without 

 influencing the total acidity. This is also borne out by the relation of dry and 

 fresh weight acidity as it is seen in the young material. The increase of acidity 

 on the basis of fresh weight is not so great as on the basis of the dry weight, or, 

 in other words, the ratio of the acid-content of the latter to that of the former 

 rises as the acidity increases. It must be admitted, however, that in the 

 series with the old turgid material this increase in ratio does not show very 

 satisfactorily, except in the highest acidity determination. However, it is 

 probable that the explanation is that which is indicated above, namely, that as 

 the acidity rises the amount of water in the tissues increases, so that the 

 concentration of the juice itself, while higher, does not rise as rapidly as does 

 the total amount of acid present. This whole question is an interesting one 

 in connection with the idea promulgated by Borowikow and mentioned 

 by Spoehr and would be worthy of closer examination than is given here. 

 According to Borowikow, there should be an increase in proportional water- 

 content as the acidity rises because of the increased hydratation of the proto- 

 plasmic colloids. The same effect, however, could be equally well accounted 

 for by the older assumption of the relation of turgor pressure to the quantity 

 of water contained by a tissue. The increased acid indicates, of course, that 

 the osmotic coefficient of the cells must be higher, and consequently their 

 ability to take in water must be greater. It is not impossible that both may 

 play their parts. It would be highly profitable to make a more detailed study 

 of this point. 



With due regard to the time of day at which the determinations were made, 

 it may be said in general that the young joints just forming show not only 

 a higher acidity than the older joints from the same plant, but also greater 

 oscillation. As an illustration of this, the lowest acidities obtained were in 

 young joints which had been exposed during the day. Mature-flaccid joints, 

 which are probably as nearly inactive as the plant ever becomes, show on the 

 average the lowest acidities and the least range of variation therein. All 

 things considered, the fully mature but still turgid joints may be taken to 

 represent the normal condition of the plant as a whole, since they form the 

 bulk of the active tissue even during the growing season, when the young 

 joints are fairly plentiful. After the young growth has attained its full 

 development during the first season it undoubtedly must lose its higher acidity 

 and greater respiratory activity, a circumstance which will be discussed later. 

 At what period the maturation of the tissue occurs would form an interesting 

 subject for investigation. The time probably comes during the dry after- 

 summer, perhaps very shortly after the cessation of the summer rains. The 

 writer has never been at Tucson at this period, and consequently has not con- 

 sidered this question. 



"Borowikow, G. A. Ueber die Ursachen des Wachstums der Pflanzen. Biochem. Zeitschr., 

 vol. XLIV, p. 230, 1913. 



