HISTORICAL NOTES. 11 



plant and that they could replace each other, and, further, that alkaloids 

 in plants could play the part of mineral bases. He ascribed certain 

 diseases of plants solely to the deficiency of mineral matter in the soil, 

 but later investigations have demonstrated that fungous or animal 

 parasites are the true causes. After about twenty years of hard fight- 

 ing the importance of Liebig 1 s mineral theory was in the main recog- 

 nized and the old humus theory abandoned. However, his opinion 

 that the mineral bases replace each other has been proved to be erro- 

 neous by the experiments of Wolff, Knop, Hellriegel, and others. The 

 indispensability of potassium was proved, especially by Friedrich 

 Nobbe, Schroeder, and Erdmann (1871), as was also the noxious char- 

 acter of lithium salts for phanerogams. Salm Horstmar and Stoh- 

 mann furnished evidence that lime and magnesia can not replace each 

 other. 



When Liebig had called attention to the necessity of certain mineral 

 constituents in plants he set his assistants and students at work to 

 analyze the ashes of a great number of plants. He published an 

 account of these analyses in his works on agriculture, but a more com- 

 prehensive review on plant ashes is given in the tables of E. Wolff/' 



These results show that the quantitative composition of the ash of 

 one and the same plant varies according to the soil upon which it is 

 grown, but that qualitatively there is no difference. This observation, 

 which led Liebig to erroneous assumptions, was properly explained 

 much later. It was found that every plant absolutely requires a cer- 

 tain minimum of each mineral nutrient, and that in most cases besides 

 this minimum it takes up not only an excess of these various com- 

 pounds, but also substances which arc perhaps useful but not abso- 

 lutely necessary for plant functions, such as sodium salts and silica. 

 In the case of potassium or calcium salts a moderate surplus is not 

 noxious. A large excess of lime taken up can be easily excluded from 

 secondary influences by transformation into oxalate or carbonate- 

 salts which are often produced by plants. Plants adapted £o saline 

 desert soils show incrustations on their leaves, which may sometimes 

 contain, in addition to chlorid, nitrate, and sulphate of sodium, more 

 than 50 per cent of calcium carbonate. 



The surplus of mineral matter found in plants — nutrient as well as 

 indifferent compounds — depends to a great extent upon the intensity 

 of the current of transpiration, which explains why herbaceous plants 

 show a higher percentage of ash for the dry matter than do the leaves 

 of woody plants. . While cabbage leaves, which have about 1>0 per 

 cent water, contain 15 to IS per cent ash for the dry matter, the leaves 

 of potatoes, clover, and grass, having 78 to 80 per cent water, contain 

 only 6 to !> per cent ash for the dry matter. In trees adapted to moist 

 .soil— for instance, Salix, Popuhts, Acer, and TiUa — the leaves contain 



aAschen AnaJysen (2 volumes), Berlin, 1871 and 1880. 



