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 



