CHAP, iv The Ash of Plants 359 



the solution ; if the solution presented is of higher con- 

 centration than this, relatively more water than salt is 

 absorbed therefrom, so that the solution becomes gradually 

 more concentrated ; if it is of lower concentration, the 

 reverse is the case and relatively more salt than water is 

 taken up. 



It was known also to the earlier physiologists that when 

 a plant grows in a medium which presents to its root hairs 

 many different salts of various metals it does not absorb 

 them equally and indifferently, or according to the osmotic 

 pressures they exercise. Again, the quantities of each com- 

 pound taken up by different roots vary considerably. This 

 was attributed to a special property of each root, which was 

 known as its quantitative selective power. The explana- 

 tion given was not very satisfactory ; it was originally 

 attributed by Mulder to the co-operation of osmotic 

 pressure and to metabolic changes following absorption. 

 The fact that deleterious salts are often absorbed in large 

 quantities, established by Phillips in 1882, shows that 

 some further explanation is desirable. 



. This selective absorption, leading to the accumulation of 

 certain ash constituents by many plants, and to the absorp- 

 tion of unequal amounts by plants growing in the same 

 soil, has been established by numerous more recent analyses. 

 The work of Wolff in 1871, Liebig in 1876, Koenig in 1889, 

 furnishes details upon this point. Goedechens' analyses of 

 various Algae made in 1845 shows that it is equally true 

 of aquatic plants. 



The explanation advanced falls short in completeness. 

 No better hypothesis was, however, advanced up to the end 

 of the century. Many physiologists came to the conclusion 

 that there exists in protoplasm a certain directive force, 

 controlling the entry and distribution of metallic salts, and 

 working together with or in antagonism to the ordinary 

 physical force of osmotic pressure. Such a process presents 



