360 The Physiology of Plants BOOK in 



certain features which recall the operations of secretion or 

 excretion. The matter was still only the subject of specu- 

 lation in 1900. 



The view of Liebig, that all absorption of salts is due 

 to the solvent action of the roots, received some apparent 

 confirmation from the observation of Sachs in 1865, that 

 roots exert a certain corrosive action on a polished gypsum 

 plate when allowed to grow in contact with its surface. 

 The explanation advanced was that of an exosmotic flow 

 of the vegetable acids of the root hairs, and was in harmony 

 with the views of osmosis that prevailed at the time. 

 Pfeffer's researches on this process, of which mention has 

 already been made, militated against this explanation. 

 Further objections to it were advanced by Czapek in 1896. 

 He showed that such acids are not excreted by root hairs, 

 and that their acidity is due to the presence of an acid 

 phosphate of potassium. Czapek said that the only acid 

 which is excreted by the roots of land-plants is carbonic 

 acid. 



The variation in nature and amount of the ash, and in 

 the quantities of its several constituents, established by 

 various authors, as we have seen, was the subject of con- 

 siderable research by Wolff, who greatly amplified the 

 information available on the subject in a memoir pub- 

 lished in 1871. In this he showed that such variation 

 depends on the plant as well as on the medium in which 

 it grows. Further proof was adduced by Grandeau and 

 Bouton in 1877 in their work on the ash of the mistletoe 

 and the host on which it was growing. Though the former 

 absorbs its salts directly from the host-plant it does not 

 take them in in the proportions in which the latter supplies 

 them. 



Three methods of investigating the problem of the ab- 

 sorption of salts were employed during the latter part 

 of the century. The first was due to Salm-Horstmar in 



