Effect of Certain Organic Acids and Amino Compounds. 55 



reproductive elements, while voluminous notices of these substance 

 and their derivatives are found in cytological literature, many of which, 

 however, need verification and analysis to make them of value. 



If protoplasm were entirely or dominantly proteinaceous, the actual 

 acidity or hydrogen-ion concentration of the sap might be taken as the 

 chief factor in maintaining the rate and determining the course of 

 hydration and growth. The predominance of the pentosans in plant 

 cells, however, offers a set of conditions much more complex than that 

 of the comparatively simple ionization of gelatine, for, as has been 

 noted, the conditions which facilitate the action of protein gels retard 

 and limit the hydration of the carbohydrate gels to an extent and in a 

 manner which depend upon the structure and character of the pen- 

 tosans present. 



It has already been shown that pentosan colloids show maximum 

 hydration capacity in the presence and under the action of certain 

 amino-compounds, a subject to which the larger part of this chapter 

 will be given. 



The actual acidity, or hydrogen-ion concentration of the sap, is 

 widely different from the total amount of acid, as some is always com- 

 bined with such bases as potassium, sodium, calcium, magnesium, 

 iron, and aluminium, making a "buffer" by which the degree of dis- 

 sociation is controlled within certain limits. This range of variation 

 as it appears in separate estimations is rather larg'e as compared with 

 variations in animals. It is necessary to bear in mind, however, that 

 a cell-mass is not uniformly acid or that the entire mass of the cell- 

 colloids is saturated with a solution of the same concentration. 



In any buffer situation, however, a lessening of the hydrogen-ion 

 concentration of the sap would be followed by increased dissociation 

 of the acid radicle of the salts, and increase of acidity beyond a certain 

 point would result in a reversal of the process. The actual acidity is 

 expressed by the negative common logarithm of the number* of dis- 

 sociated hydrogen ions given as the value of Sorensen's symbol pH. 

 This may vary from 3.9 to 5.7 in various succulents examined by 

 Jenny Hempel, and may approach neutrality at pH-7 in some cases. 

 A singular instance of wide difference between actual and total acidity 

 is offered by lemon fruits, the sap of which has an actual acidity of 

 pH = 2.3, which is about one-tenth the total acidity, which may be 

 expressed as about 0.05 to 0.06 N. 1 



The variations in the hydrogen-ion concentration of the cell-sap 

 and the determination of the agencies which may cause such changes 

 offer a most inviting field for research. 2 In a recent paper, R. B. 

 Harvey has described some extremely interesting changes in the 

 as determined by potentiometer methods, of cabbage leaves in acidity, 



1 Hempel, Jenny. Buffer processes in the metabolism of succulent plants. Compt. Rend, 

 d. trav. d. Lab. Carlsberg, 13: No. 1. 1917. 



2 Haas, A. R. The reaction of plant protoplasm. Bot. Gazette, 63: 232. 1917. 



