THE NUTEITION OF THE EOOT 35 



% 6. What part is played by the subsidiary nutritive sub- 

 stances and the occasional admixtures to the nutritive 

 salts ? 



Sodium must be looked upon as a subsidiary food sub- 

 stance, though it is absent only in very few plants, and is 

 taken up by some groups of plants (shore plants) in enor- 

 mous quantities in the form of sodium chloride (common 

 salt), and is found in their tissues together with potassium. 

 But in spite of these two elements being closely allied, they 

 cannot in any way replace one another. Thus, if a plant 

 is without potassium, a supply of sodium will be of no use 

 to it. The roots take up the very soluble sodium salts in 

 •great quantities if they require the acids with which the 

 sodium is combined, and store up the sodium in another 

 form as a waste product. But we gather that this sub- 

 stance is of little importance in plant economy from the fact 

 that even those plants which always present a large amount 

 of sodium in their ash, such as the saline plants of the 

 shores, can be cultivated to perfect development in a soil 

 free from sodium. 



Silieium, which is present in all soils in the oxidised form 

 ■oi siliea, behaves just like sodium. Silica is generally found 

 in the soil in the form of crystalline (insoluble) substance, 

 but it occurs also in an amorphous soluble form, and can 

 in this condition be easily absorbed by the roots. Being so 

 widely distributed, it is found very generally in plants, 

 and, like lime, and often together with it, it is used to 

 strengthen the cell-walls. Such cell-walls are often so 

 strongly impregnated with silica, that in some cases (Uqui- 

 jsetwri) these plants may be used for polishing purposes. The 

 stems {culms) of our cereals and the leaves of sedges are 

 provided with epidermal cells so stronly protected with silica 

 -that a hand passed over them will be severely cut. The 

 enormous quantity of silica contained in grasses and sedges 

 was formerly adduced as a proof of the necessity of silica for 

 the strengthening of the cell-walls. But now that it has been 

 possible to cultivate cereals in solutions devoid of silica just as 

 ^ell as in the open field, this inference breaks down. The laying 



