SELECTIVE POWER OF ROOTS. 287 



This opportunity may be taken of remarking that the roots possess a sort 

 of selective power, as it has been called, as to quantity at any rate, even if not 

 as to quality. That is to say, the roots absorb dissolved matters of the most various 

 kind, it is true, even those which are injurious, and thus are not able to reject any ; 

 but on the other hand it was shown by De Saussure, and has been confirmed by more 

 recent investigations, that the roots absorb the nutritive salts at their disposal by no 

 means in the same quantitative proportions as those in which they are met with in the 

 mixed solution. If, for instance, the solution contains more calcium than potassium, 

 the subsequent analysis of the plant-ash may yield more potassium than calcium, and 

 so forth. This fact, depending upon a power of quantitative selection on the part of 

 the plant, is evident in the thousands of ash analyses which have been made of the 

 most various plants ^ developed in the open. Plants of different species which have 

 grown close together on the same soil, or in the same water, exhibit totally different 

 combinations in their ash, — the one having much, the other little, of calcium com- 

 pounds; in the one magnesium and calcium predominate, in the other potassium, 

 compared with the quantity in which these substances are contained in the soil or (in 

 the case of water-plants) in the water. The latter is particularly conspicuous in marine 

 plants. While sea-water contains 3°/q of common salt, and only very small quantities 

 of salts of potassium, magnesium, and calcium, the salts last named nevertheless pre- 

 dominate in the ash of these plants, and it contains relatively but little of the common 

 salt of the sea-water. Plants thus take what they require from a mixtvu-e of salts, 

 without particular reference to the composition of the nutritive mixture. But, again, 

 even this does not exactly occur in the sense that they only and exclusively take up 

 just so much of each material as is necessary for nutrition ; since in the ash of plants 

 of the same species are found more or less potassium, calcium, magnesium, phos- 

 phoric or sulphuric acids according as they have grown on this or that soil. The 

 composition of the ash of a species of plant is thus not specifically constant, but is 

 dependent, at least in part, upon the composition of the soil or the water. Least 

 variable in this respect are, of course, the fruits and seeds : in these are collected 

 only the materials specifically necessary for the seedling, while in the shoots and 

 roots superfluous matters also may be lodged. 



The question still remains, however, whether all the salts which we have added to 

 our nutritive solution are actually necessary for the plant. These salts were, potassium 

 nitrate, calcium sulphate, and magnesium and calcium phosphates; we have already 

 learnt that the iron salt is indispensable, and recognised the incidental significance 

 of the sodium chloride. The question raised may be in the first place simplified by 

 regarding as the essentials, not the salts named as such, but the elements contained 

 in them. As numerous investigations in this direction show, it is simply necessary 

 that the nutritive mixture should contain the elements potassium, calcium, magnesium, 

 iron, phosphorus, and sulphur, in suitable neutral combinations containing oxygen. 

 For reasons the explanation of which would here carry us too far, however, it is best 

 to choose the salts given in our table ; and it is at the same time certain that land- 

 _ plants, as well as water-plants, as a rule, find and take up just these very salts in their 

 environment, since potassium nitrate, gypsum, magnesium sulphate, calcium phosphate, 



' Emil Wolff, ' Aschmanalysen von landwirthschaftlichen Producten,' Berlin, 1871. 



