THE CONSTITUENTS OF THE ASH OF PLANTS 185 
perhaps, to the plants of the sea-shore, m 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 thelr weak stems, and 
