296 NATURE AND PROPERTIES OF SOILS 



The plant might thus facilitate special conditions and aid ma- 

 terially in the preparation of its own nutrients. 



Sachs, 1 and later other investigators, grew plants of various 

 kinds in soil and other media in which was placed a slab of 

 polished marble or dolomite or calcium phosphate, covered 

 with a layer of washed sand. After the plants had made 

 sufficient growth the slabs were removed, and on the surfaces 

 were found corroded tracings, corresponding to the lines of 

 contact between the rootlets and the minerals. 



Czapek 2 repeated the experiments of Sachs, using plates 

 of gypsum mixed with the ground mineral that he wished 

 to test, and this mixture he spread over a glass plate. Cza- 

 pek found that, while plates of calcium carbonate and of 

 calcium phosphate were corroded by the roots, plates of alu- 

 minum phosphate were not. He concludes that if the tracings 

 are due to acids excreted by the roots, these acids must 

 be those that have no solvent action on aluminum phos- 

 phate. This would limit the excreted acids to carbonic, 

 acetic, proprionic, and butyric. By means of micro-chem- 

 ical analyses of the exudations of root-hairs grown in a 

 water-saturated atmosphere, Czapek found potassium, mag- 

 nesium, calcium, phosphorus, and chlorine in the exudate. He 

 concludes that the solvent action of roots is due to acid salts 

 of mineral acids, particularly acid potassium phosphate. He 

 has not proved, 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 is doubtful. This ob- 

 jection makes the possible exudation of organic and inorganic 

 acids somewhat questionable. 



Molisch 3 found that root-hairs secrete a substance having 



1 Sachs, J., Auflosung des Marmors durch Mais-Wurzlen; Bot. Zeitung, 

 18 Jahrgang, Seite 117-119, 1860. 



"Czapek, J., Zur Lehre von den Wurzelausscheidung ; Jahrb. f. Wiss. 

 Bot., Band 29, Seite 321-390, 1896. 



8 Molisch, H., uber Wurzelausscheidungen und deren EinwirTcung auf 



