180 VEGETABLE PHYSIOLOGY 
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 
consequently to the maintenance of their vertical position. 
This is, however, not the case; their rigidity is dependent 
on the degree of development of their harder tissues, and 
the absence of silica makes but little difference to them. 
Silicates, when added in quantity to the soil in which green 
crops are growing, have no marked effect upon the amount 
of silica which is subsequently present in the straw. It is 
uncertain whether the silica enters into the metabolism of 
