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'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 potassa was proved, especially by Friedrich ISTobbe, 

 Schroeder, and Erdmanu (1871), as was also the noxious character of 

 lithium salts for Phanerogams. But the real significance of the bases 

 in the plant cells has not been cleared up, and, as a botanist has 

 exx)ressed it, the solution of these problems must form an important 

 final goal for every plant physiologist. 



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- 

 Ijrehensive 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 certain 

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

 minimum it takes up not only an excess of these various compounds, but 

 also substances which are perhaps useful but not absolutely necessary 

 for plant functions, such as sodium salts and silica. In the case of potas- 

 sium or calcium salts a moderate surplus is not noxious. A large 

 excess of lime taken up can be easily excluded from secondary influ- 

 ences by transformation into oxalate or carbonate — salts which are 

 often produced by plants. Plants adapted to 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 fouud 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 90 i^er cent 

 water, contain 15 to 18 per cent ash .for the dry matter, the leaves of 

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

 only G to 9 per cent ash for the dry matter. In trees adapted to moist 

 soil, for instance, Salix, Fopulus, Acer, and Tilia, the leaves contain more 

 water and also generally more ash for the dry matter than do the leaves 

 of trees in which transpiration goes on more slowly, such as Quercus, 



Ascbeu Analysen (2 volumes), Berlin, 1871 and 1880. 



