THE SOLVENT ACTION OF ROOTS UPON 

 THE SOIL PARTICLES 



By A. D. HALL, M.A. 



Director of the Rothamsted Experimental Station 



Though it has always been recognised that the roots of plants 

 in the main derive their nutriment from substances dissolved in 

 the water within the soil, yet the possible direct solvent action 

 of the roots themselves upon the solid materials has long been 

 a debated question amongst plant physiologists and agricultural 

 chemists. The problem has been attacked from several distinct 

 points of view, and the investigations find applications in one 

 or two rather unexpected directions. 



The starting-point in the discussion may be taken to be 

 the classical experiment of Sachs (Bot. Ztg. i860, 117), repeated 

 nowadays in every botanical laboratory, in which a slab of 

 polished marble is placed vertically in the soil of a pot 

 carrying a growing plant. When it is removed after a few 

 weeks' growth of the plant it will be found to be etched 

 wherever the network of fine roots has been in contact 

 with the smooth surface. Sachs further found that such 

 etched figures were produced upon plates of dolomite, mag- 

 nesium carbonate, and calcium phosphate, but not upon sili- 

 cates ; while a gypsum plate showed a reversed effect of 

 raised lines beneath the roots, which seemed to have pro- 

 tected the original surface from the solvent action of the 

 water in the soil. Since it is well known that the expressed 

 sap of the roots of most plants contains a certain amount of 

 free organic acid, these etching effects have often been put 

 down to the exudation of the acids in the sap through the thin 

 cell walls of the roots. Sachs, however, regarded the carbon 

 dioxide, which is always being excreted by the plant's roots, 

 as the origin of the corrosion. He pointed out that the 

 substances attacked were all soluble in water containing carbon 

 dioxide, which alone would be able to effect the phenomena 

 observed. Again, the soft cell membrane of the root hairs, 

 which is in such intimate contact with the soil particles, must 

 contain a relatively strong solution of the carbon dioxide that 



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