126 PLANT PHYSIOLOGY 



Plants are often very specific in their pH requirements, and two 

 species of the same genus may have very different demands. 

 Galium Sylvester, e. g., requires neutral or slightly alkaline soil, 

 while G. saxatile grows best on acid soils. Many similar examples 

 could be cited. Seed germination and growth have an optimum 

 pH value, and there is hardly a living process for which there is 

 not an optimum H + ion concentration. Also different organs of 

 the same plant may show differences in pH value; thus Gustafson 

 found in corn, sunflower, and pumpkin that the H + concentration 

 increased from base to tip. During the last twenty years papers 

 by the thousands have been published on the minimum, optimum, 

 and maximum pH values of the various life processes and phe- 

 nomena, and this "fad" has reached such heights that it has been 

 proposed to call the doctor's degree in physiology a pH D ! From 

 this mass of data, however, many valuable and practical results 

 have been obtained. 



Changes in soil pH may influence the plant (a) either by directly 

 affecting the protoplasm of the root cells and the cell processes 

 therein, or (b) by changing the relations of the various materials 

 present in the soil. The change in cell permeability is an example 

 of the first kind of effect, while the effects upon base exchange 

 would be an example of the second. The effect of acidity upon the 

 solubility of aluminum is another instance of the indirect effects 

 of acids. When a substance is added to the soil, it is often difficult 

 to know, therefore, whether the plant has been directly or in- 

 directly affected. 



Differences in the absorption rate of the two ions of a salt may 

 make a great change in the pH value of the solution. This point 

 has been mentioned in the discussion of nitrate vs. ammonia nu- 

 trition of plants (Chap. IX). If bases are left in the soil, it will 

 tend to become alkaline (as when nitrates are used); while if 

 acid groups are left, the soil, of course, will become more acid 

 (as when ammonium salts are used). In general, plants tend to 

 absorb ions so that the result is to bring the soil reaction nearer 

 to neutrality. This action is further favored in certain cases by 

 the active secretion of substances by the roots. Thus if a plant 

 were grown in an acid soil, the soil reaction in the vicinity of 

 the plant could be made more nearly neutral by absorbing the 

 acid radicles from the soil or by actively excreting alkaline sub- 

 stances. Jacobson found that wheat plants 100 days old changed 



