THE CONSTITUENTS OF THE ASH OF PLANTS 185 



perhaps, to the plants of the sea-shore, in which it pro- 

 motes their peculiar succulence. 



If we compare the influence of potassium, sodium, and 

 calcium on the development of a crop of herbage plants, 

 we find that the presence of potassium leads to a develop- 

 ment of stems, flowers, and fruit, or to what may be regarded 

 as the maturing of the plants ; while in the absence of 

 sufficient potassium and the presence of calcium and sodium 

 vegetative growth is more directly favoured, but the crop 

 remains backward and immature. 



There is a possibility that all these metals serve another 

 purpose as well as some particular functional one. We 

 have seen that the nitrogen which the plant obtains is 

 derived from the soil, being most favourably supplied by 

 the latter in the shape of nitrates. In the soil the nitric 

 acid is combined most frequently with the metals under 

 discussion, and a not inconsiderable quantity of the latter 

 may be taken up solely for the sake of the nitrogen which 

 they can thus carry into the plant. The varying amounts 

 of sodium and calcium which plants contain have been 

 found to bear a certain relationship to the amounts of 

 their compounds, which occur in the particular soils in 

 which the plants have been growing. When calcium and 

 sodium nitrates are taken up for the sake of the nitrogen, 

 they are probably decomposed by the organic acids formed 

 in the plant, and the nitrogen is made to enter into further 

 combination, leading to the construction, possibly of amino- 

 or amido-acids, and eventually of proteins. 



Of the other elements which are included with sodium 

 in this group, silicon is one of the most prominent. It is 

 absorbed almost entirely in the form of silicates of potas- 

 sium and sodium, the latter combination being the prin- 

 cipal one. It is difficult to say what purpose it serves. 

 It is usually found deposited in the epidermal cell-walls, 

 and as the grasses and the horsetails contain it in greatest 

 abundance, it has been suggested that its utility consists in 

 its contributing to the rigidity of their weak stems, and 



