134 LECTURE VIII. 



The functions with which potassium appears to be espe- 

 cially connected in plants containing chlorophyll, is that of 

 the formation of organic substance. Thus Nobbe found, in a 

 number of water-cultures, that a plant not supplied with salts 

 of potassium grew but little (Fig. 21, 2, 3), did not increase 

 in weight, and that the amount of starch in the plant was 

 very small, being represented only by a few grains in the 

 chlorophyll-corpuscles of the leaves. On the addition of 

 potassium chloride the starch-grains became more numerous 

 in the chlorophyll-corpuscles, and made their appearance also 

 in the tissues of the stem. It is, however, not known what is 

 the precise significance of potassium in relation to the forma- 

 tion of organic substance. 



Liebig was of opinion that potassium played an important 

 part in the distribution of carbohydrates throughout the 

 plant, but this has not been adequately confirmed. It ap- 

 pears rather that the observed facts upon which this view was 

 based, point to the effect upon the plant not of the potassium 

 itself but of the particular form in which it was presented. 

 This point will be again touched upon when we are consider- 

 ing the use of chlorine in the plant. 



That potassium bears some important relation to the for- 

 mation and to the storing-up of carbohydrates is shewn by 

 the fact that the organs in which these processes are taking 

 place, such as leaves, seeds, tubers, etc., are those parts of 

 plants which are richest in this element. 



Potassium occurs in considerable quantities in plants in combination 

 with organic acids. 



Naegeli has observed that Caesium or Rubidium can replace potassium 

 in the food of certain Fungi (Moulds, Yeast, Bacteria). 



Calcium. The compounds in which calcium is usually 

 supplied to plants are the sulphate, phosphate, nitrate, and 

 carbonate, the last undergoing decomposition in the process. 

 When absorbed in these forms it contributes to the well-being 

 of the plant, whereas when it is absorbed in the form of 

 chloride the effect is unfavourable. 



The precise use of calcium to plants is unknown. It 

 cannot be replaced in the food of plants which contain chlo- 



