8o 



METABOLISM 



In addition to the problem as to how the constituents of the ash enter the 

 plant we have the further question as to whether they are of value to it or 

 merely accessories accidentally introduced along with the water. Senebier 

 (1800) and Saussure (1804) have already shown that certain minerals are 

 essential to the plant as food-stuffs ; this view Liebig (1840) supported very 

 strongly, and owing to his authority it received general acceptance, although, 

 strictly speaking, it was not exactly proved till later. The methods employed 

 for this purpose are two in number. Both were intended, at the same time, 

 to show whether all or which of the ash constituents found in the plant were 

 essential. Important experiments for this purpose were first carried out by 

 Prince Salm-Horstmar (1856), who employed the first method. He cultivated 

 the plants, following the example set by Wiegman and Polstorff (1842), 

 in insoluble artificial soils, to which were added the materials which were to 

 be investigated. He used, e. g., soil composed of pulverized rock crystal and 

 carbon obtained from candy sugar, whilst Wiegman and Polstorff worked with 

 platinum filings and sand. Salm made out that silicon, phosphorus, sulphur, 

 potassium, calcium, magnesium, iron, and manganese were essential to the 

 normal development of oats, but he was doubtful as to the significance of 

 chlorine. Although his results could not be completely substantiated, still they 

 were very valuable in one point, inasmuch as they proved that sodium, though 

 never absent from the ash of plants, was not to be included among the 

 essential elements of oats (although Salm believed it to be essential in other 

 cases). This fact is all the more remarkable when one remembers that sodium 

 has important functions to perform in the higher animals. 



The other method — the so-called water-culture method — is of the greatest 

 value for our purpose. Although Saussure (1804) early in the century grew 

 Bidens and Polygonum in water, Sachs (i860) and Knop (i860) were the first to 

 cultivate, experimentally, land plants in such a way that their roots, immersed 

 in a watery solution of various salts, could supply their requirements so far 

 as inorganic salts were concerned, and proved that the plants cultivated in 

 this way showed a large increase in their dry weight. An increase in dry 

 weight, especially a large increase, is a valuable criterion of the success of such 

 a culture, but we should fall into serious error were we to conclude from this 

 that a plant grows only if it be supplied with all the necessary food materials. 

 Growth can take place without increase in dry weight, and indeed without 

 the absorption of water. Again, we should be totally wrong were we to 

 conclude from the fact that plants, without taking up nitrogen, can reach 

 a weight three and a half times that of their seed (Boussingault, i860), that 

 nitrogen was not necessary for growth. From the observations of many in- 

 vestigators it has been shown that maize can develop under the most favourable 



