358 The Physiology of Plants BOOK HI 



on the interaction of many elements with the protoplasm, 

 interaction which may vary from time to time and from 

 place to place according to the changes of the environment 

 or the automatic readjustments going on in the living 

 substance. The influence of varying quantities of the 

 mixture of elements may be considerable. Correlations of 

 the functions of the plant, or the abnormal performance 

 of one or more of them under the influence of disturbances 

 of quantitative relations, may obscure the action of any 

 element or group of elements in whatever combinations it 

 or they may be existing. It is therefore hardly to be 

 expected that much progress will be made in this direction ; 

 probably all the essential elements of the ash play many 

 parts, all co-ordinated by the living substance of the 

 organism. 



We have already examined the development of knowledge 

 of the absorption of water by the roots of plants. This 

 process is associated very closely with the absorption of 

 the elements of the ash, as was established long ago by 

 De Saussure, whose law of absorption expressed the views 

 current up to the opening years of the period under review. 

 Liebig was a little more pronounced than De Saussure in 

 his opinions, for he taught in 1858 that terrestrial plants 

 absorb all their ash constituents from the soil by means 

 of the solvent action of their roots. 



De Saussure's law was re-investigated by Wolff in 1864, 

 by Knop, in 1859 and 1864, and by Biedermann in 1867. 

 The result of these researches, of which those of Wolff 

 were the most complete, was to establish that De Saussure's 

 law does not hold universally, but deals rather with a special 

 case of a more general rule. The latter, as established in 

 these investigations, is that for every salt capable of 

 being absorbed there is a certain degree of concentration 

 at which the proportion of the amount of salt absorbed to 

 that of the water absorbed with it is the same as that of 



