THE ABSORPTION OF MINERALS 195 



in metabolism but even some that were distinctly 

 injurious. 



Liebig had expressed the view that absorption of 

 minerals was due to the solvent action of roots, a con- 

 ception that seemed to be supported by Sachs's statement 

 in 1865 that roots were able to corrode polished slabs 

 of marble, a phenomenon which was attributed to the 

 exosmosis of certain organic acids by the roots. By the 

 end of the century this idea had been completely exploded 

 by Czapek's work published in 1896. 



In all the enquiries made during the later years of the 

 century as to the functions of the different soil constituents 

 two methods of investigation had been followed. One 

 was that introduced by Salm-Horstmar in 1860, viz. 

 to compound an artificial soil of insoluble constituents 

 and then to add to it solutions of soluble salts known 

 to be of service in plant nutrition ; the other was that 

 introduced by Sachs and Knop in 1860, and improved 

 by Nobbe in 1862, known to you as the " water-culture " 

 method. The results obtained seemed to show that, 

 in addition to carbon, hydrogen, oxygen, and nitrogen, 

 the metals potassium, calcium, magnesium, iron, 

 manganese, and the non-metals phosphorus, sulphur, 

 silicon, and, doubtfully, chlorine, were essential to all 

 green plants. Salm-Horstmar affirmed that sodium 

 was not an essential ash constituent. Salm's results 

 were in the main confirmed by Birner and Lucanus 

 in 1866, save that they claimed that sodium was essential 

 while silicon and manganese were not. 



I think it is scarcely worth while to recapitulate all 

 the attempts that were made to attribute a special 

 function to each element, for no sooner was a statement 

 made by one investigator than it was contradicted by 

 another. Take iron for instance. Gris, in 1843, showed 

 that this metal was essential to the formation of chloro- 

 phyll. This led to the belief that iron was a constituent 

 of the pigment, and some chemical physiologists, e.g 



