ROOT EXCRETIONS AND SOLUBILITY 289 



num phosphate were not. He concludes that if the trac- 

 ings are due to acids excreted by the plant-roots, the 

 acids so excreted must be those that have no solvent 

 action on aluminum phosphate. This would limit the 

 excreted acids to carbonic, acetic, propionic and butyric. 

 Czapek also replies to the argument that the acids pro- 

 ducing the tracings must be non-volatile ones, because 

 of the definite lines made in the mineral, by stating 

 that the excretion of carbon dioxide alone would be 

 sufficient to account for the observations, as it dissolves 

 in water to form carbonic acid, and that carbonic acid 

 is always present in the cell-walls of the root epidermis, 

 from which it does not readily exude. 



Czapek has also shown that liquids having an acid 

 reaction exude from root-hairs, and he attributes the 

 reaction to the presence of acid salts of mineral acids, 

 having found potassium, phosphorus, magnesium, cal- 

 cium and chlorine in this exudate. He has not proven, 

 however, that the exudations were not from dead root- 

 hairs, or from the dead cells of the root-cap. In either 

 case they would have some solvent action, but whether 

 sufficient to make them of importance it is impossible 

 to say. 



Kunze, who followed up this work, discredits the 

 theory of excretion of acid salts of mineral acids, and 

 attributes the corrosive action of roots to organic acids. 

 In his experiments with 200 species, he found that many 

 plants do not excrete enough acid to be detected by 

 litmus. He attributes to fungi much the greater activity 

 in this respect, and considers them more important in 

 disintegrating the soil than are the higher plants. 



