46 Hydration and Growth. 



tration of the combined solution should be equivalent to that of the 

 salt alone. The combination of citric acid and potassium nitrate is 

 open to some objection, but the effects described are similar to those 

 obtained by the use of potassium chloride and hydrochloric acid. 



The total acidity of pure juice of fresh material at Tucson varied 

 from 0.0280 in the morning to 0.0232 per centimeter 0.01 N of sodium 

 hydoxid at 4''30" p. m. Probably some of the acid was broken up 

 during the drying, but the cell colloids would still be decidedly acid. 

 The hydrogen-ion concentration in another species of Mesemhryan- 

 themum determined by Lakmoid tests and electrometer measurements 

 by Hempel gave values of pH — 4.8 to 5.2 for the first and 4.61 to 4.84 

 by the second method. 



The matter was given further test by taking sections of young leaves 

 in a flaccid condition and measuring the total swellings, which are 

 given in table 35. 



Table 35. 



p. ct. 



Water 22 



Potassium nitrate, 0.01 M 22 



Potassium nitrate, citric acid, 0.01 N 17 



Citric acid, 0.01 N 17 



Sodium hydroxid 11 



The swelling in potassium nitrate agreed with that of the dried 

 material in being nearly equivalent to that in distilled water, and less 

 in acid salt solution, but the swelhng in acid is less, the proportionate 

 swelling in hydroxid being about the same. 



The effects of a similar series of reagents were tried upon disks from 

 growing joints of Opuntia, with the results given in table 36. 



Table 36. 



p. ct. 



Distilled water 11 



Potassium nitrate, 0.01 M 9 



Potassium nitrate, citric acid, 0.01 N 10 



Citric acid, 0.01 N , 9 



Potassium nitrate, potassium hydroxid, 0.01 M 10 



The relations here are different in character from those exhibited 

 by the material previously examined, but the departures are so small 

 that no safe conclusion may be founded on them. 



The incorporation of any salt with a colloid in the disperse phase 

 would of course allow the formation of adsorption compounds to an 

 extent and of a kind not possible when the salt enters the hydrating 

 gel in a solution. The hydration of such a salted colloid might be 

 expected to take place at a different rate and to a total varying from 

 that of the unsalted colloid with unsatisfied chemical affinities and 

 under the conditions of surface tension which would prevail.-' 



' Loeb, J. The similarity of the afttion of salts upon the swelling of animal membranes and 6f 

 powdered colloids. Jour. Biol. Chem., 31: 343. 1917. 



