86 Journal of Agricultural Research voi. xix. No. a 



It will be noted that the actual numerical value of the change in reac- 

 tion is closely related to the stability of a given culture as indicated by 

 the slope of the electrometric titration curve at the point representing 

 the composition of the solution. There appears, however, to be a general 

 tendency for the more acid cultures of the series to become slightly less 

 acid while the more alkaline members tend to become slightly less alkaline. 

 The conditions were not such as to permit accurate determination of 

 the change in total titrable acidity or basicity produced by growth. 

 However, if the points on the electrometric titration curve (fig. 2 , A) , corre- 

 sponding to the reaction of each culture before and after growth of seed- 

 lings, are projected upon the horizontal axis representing quantity of 

 total alklai added, it is found that in cultures 2 to 7, inclusive, there were 

 no large differences in the quantitative value of the change in reaction — 

 that is, a change in reaction of 0.41 Ph in culture 3 or 4 does not neces- 

 sarily correspond to a greater change in total acidity than a change of 

 0.08 Ph in culture 5. The exact cause of the change in reaction,whether 

 due to root excretions, to selective ionic absorption, or to other factors, 

 was not determined. The results obtained agree with those of Panta- 

 nelli (20), who found a general tendency for plants grown in solution 

 culture to regulate the reaction towards that most favorable to growth. 

 The results agree also with the more recent work of Hoagland (77, 12), 

 who found that barley grown in partial and complete nutrient cultures 

 caused the reaction to approach that of approximate neutrality. On 

 account of the difference in conditions the foregoing data are not neces- 

 sarily contradictory to the results of Breazeale and LeClerc (7), who grew 

 wheat seedlings in solutions of the single salts K2SO4, potassium chlorid 

 (KCl) , and NaNOg and found a development of acidity in the potassium 

 salts and of basicity in NaNOg apparently due to selective ionic absorp- 

 tion. However, in the more recent work of Hoagland {12), who grew 

 barley plants in single salt solutions of KCl, K2SO4, MgSOi, potassium 

 phosphate (K3PO4), ammonium chlorid (NH4CI), and NaNOg, he found 

 that— 



in no case was a condition either of excessive OH ion or H ion concentration pro- 

 duced, although absorption had been active. The acid reaction when present was 

 due to slightly dissociated acids, usually carbonic, or to acid salts in the case of NH4CI 

 solution. Possibly in some cases organic acids were formed. 



In this connection it should be mentioned that Haas (7) grew wheat 

 seedlings in distilled water and found no change of reaction, measure- 

 ments being made after carbon dioxid had been removed. 



POSSIBLE INFLUENCE OF FACTORS OTHER THAN REACTION 



While it seems probable that the variations observed in the growth 

 of the seedlings under the range of reactions employed were the direct 



