SOLUTION OF MINERALS 69 



acids are secreted by roots, but the proof is in most cases 

 insufficient. Goebel (1893) showed that roots of cress and 

 barley secrete formic acid, but this is stated by Czapek to 

 be in the form of an alkaline salt ; Schulow (19 13), who has 

 demonstrated the secretion of malic acid by peas and maize, 

 is doubtful whether it is in the form of a free acid or of a 

 salt. Recently Haas (1916) has shown by measurements 

 of hydrogen ion concentration that carbon dioxide accounts 

 for the increase of acidity produced by roots of wheat and 

 maize. A great difficulty in the way of exact demonstration 

 is that of raising plants in strictly sterile conditions. There 

 is no doubt that in many cases the acids found have been 

 bacterial products ; of course, in nature, such acids may have 

 an important action which is not, however, to the credit of 

 the higher plant. This may help to explain such facts as 

 the utilisation of aluminium phosphate, which is insoluble 

 in carbon dioxide water. 



But here another possibility may be noted — that given 

 by the differential absorption of the two ions of a salt. 

 If PantaneUi shows, for example, that, from a solution of 

 potassium sulphate, the chick-pea absorbs iSz mg. of 

 potassium and only 0-51 mg. of the sulphate radical, this 

 means that a considerable surplus of free sulphuric acid is 

 left in the soil and will exercise the solvent action of a 

 strong acid. On the other hand the acid radical is more 

 extensively absorbed from some salts, such as calcium 

 nitrate, and this results in an increased alkalinity of the 

 medium. Redfern (1922) has shown that in very dilute 

 solutions of calcium chloride the two ions are absorbed 

 almost equally, though in stronger solutions the calcium 

 is absorbed more readily. Jones and Shive (1922) find 

 that in a complete nutrient solution the acidity tends to 

 diminish under the action of roots, except when ammonium 

 salts are present. We have already referred to other 

 sources of acids in the soil. 



Submerged Plants.— The root systems of submerged 

 plants have been described, and it has been found that they 

 are active in absorbing water. That this should be a 



