192 PART III. PHYSIOLOGY. [ 45 



be indispensable in the case of one plant only, the Buckwheat 

 (Polygonum Fagopyrum). 



Iron, though it is met with in very small quantities, is absolutely 

 necessary for the formation of chlorophyll. The leaves produced 

 by plants which are not supplied with iron during their growth, 

 are white so soon as their own store of iron is exhausted ; 

 these leaves, which are said to be chlorotic, become green in con- 

 sequence of the formation of chlorophyll if the soil be supplied with 

 iron, or even if their surface is washed with a very weak solution 

 of iron. 



Potassium. Unless the soil contains potassium-compounds, the 

 assimilation of carbon dioxide by plants possessing chlorophyll does 

 not go on, as is shown by the fact that, under these circumstances, 

 the plant does not increase in dry weight. Potassium-salts are 

 especially abundant in those parts of the plant which are rich in 

 carbohydrates such as starch and sugar, as in potatoes, beet-roots, 

 and fruits. 



Calcium and Magnesium have been shown to be necessary to the 

 normal development of plants : they are absorbed as nitrates, phos- 

 phates and sulphates, and thus serve as bases for the absorption of 

 these other important elements. Little is known as to their direct 

 use : they may be of importance in neutralising the organic acids 

 (especially oxalic) formed in the plant. 



The distinction of the essential from the non-essential elements 

 has been arrived at by the method of water-culture, which consists 

 in growing plants from the seed with their roots in a solution of 

 various salts in distilled water. By varying the salts in the 

 solution, and observing the effect of the change on the health of the 

 plant, the relative importance of the different elements can be 

 ascertained. The following are examples of solutions containing all 

 the essential elements : 



1. 2. 



Potassium nitrate Calcium nitrate 



Calcium phosphate Potassium sulphate 



Magnesium sulphate Magnesium phosphate 



Ferric chloride Ferric chloride. 



In these two mixtures, as well as in others of the same acids and 

 bases which might be formulated, all the essential elements are in- 

 cluded in forms suitable for absorption ; the proportion of mixed 



