61 



we can therefore understand why a plant dies in the 

 absence of water. 



I have no hesitation in advancing the theory that 

 the same portion of Potash does its work over and 

 over again ; that is to say, the same quantity of 

 Potash, after dissolving a minute portion of Silica, 

 conveys it through all the sap cells, from the root to 

 the utmost extremity of the leaves, and, after 

 depositing the required quantity, returns for a 

 fresh supply to be deposited as before. There 

 is no doubt that a portion of the alkali itself is 

 retained within the cells in the course of these 

 journies, but the greater portion is returned to the 

 soil. 



This theory is based upon many facts and 

 observations. From an analysis of wheat-straw, I 

 find that, in every 100 parts of ashes, there are 

 present 22 parts of Potash to every 61 parts of 

 Silica ; and as we know that it requires at least 

 two parts of Potash to dissolve one part of Silica in 

 combination, we have to account for 100 parts of 

 Potash which are wanting. Where is this balance 

 gone to ? It cannot have evaporated. It must be 

 concluded, then, that the same quantity has been 

 employed repeatedly in carrying through the system 

 of the plant the different minerals otherwise inso- 

 luble in water. Another phenomenon has been 

 observed which tends to strengthen this conclusion, 

 namely, that we find the largest percentage of 



