ESSENTIALS OF PLANT UFE 13 



is covered with black paper to exclude light and so prevent the 

 growth of green algae, which interfere with the success of the 

 experiment. The nutrient solution is replaced by a fresh solu- 

 tion every few days, to avoid injury to the plants by change in 

 the chemical composition of the solution. Absorption of excess 

 of acid or basic radicals by the plants, leaves the liquid acid or 

 alkaline, according to its previous composition. 



Where the plant grows well, reaches a good size, and produces 

 seed, the element absent from the solution is not essential. 

 Where the plant makes a very poor and imperfect growth, and 

 produces only a few seed, or none at all, the element absent from 

 the solution is essential. Since the seed always contain a certain 

 quantity of the essential elements, some growth of the plant is to 

 be expected. 



The results of one series of experiments, are not, of course, con- 

 sidered final. In experimental work, the work of one man has 

 usually to be confirmed by that of others, before it receives gen- 

 eral acceptance. 



The method of experiment known as "water culture" 1 , is 

 suitable for experiments in which the material supplied to the 

 roots must be accurately controlled. It is impossible to secure a 

 soil or a sand from which plants will not obtain some mineral 

 material. Weigmann and Polstorff, as we have seen, found that 

 plants extracted ash from sand which had been exhausted with 

 strong acids, and other workers have had similar experiences. By 

 using pure water and pure salts, we know exactly what material 

 is presented to the roots of the plants. The plant may take up a 

 small portion of the silica from the glass when the experiment is 

 conducted in glass vessels, but this is not usually of importance. 

 If necessary, vessels of platinum or of paraffin, can be used. 



The Essential Elements. It has been found by experiments 

 such as those described above, that the following elements are, 

 without doubt, essential to the life and growth of plants : 



The four metals : potassium, calcium, magnesium, and iron. 



The three non-metals : nitrogen, sulphur, and phosphorus. 

 1 Knop, Sachs and others, Jahresber. Agr. Chem., 1861, pp. 126, 136. 



