ioo THE LIFE OF THE PLANT 



iron that brings back healthy colour to a faded cheek 

 brings back the natural green colour to a white leaf. 



Results similar to those just described in connection 

 with nitrogen, potassium and iron, can be obtained in 

 the same way in respect of phosphorus, sulphur, chlorine, 

 lime, and magnesium. All these substances have proved 

 to be indispensable for the nutrition of the plant, which 

 sooner or later perishes without them. 



But among the constituents of the ash of the plant 

 we find silicon. Silicon, together with oxygen, forms 

 silica, which in a pure form occurs in nature as rock 

 crystal, and a little less pure as quartz, white sand, 

 etc. The same silica forms the main constituent of 

 glass. This silica is also found in many plants, in 

 their cell-walls, making them brittle like glass ; 

 if we burn such a cell we are left with a glassy 

 skeleton, which under the microscope preserves the 

 outward form of the living cell in its minutest 

 details. By very unpleasant experience every one has 

 had opportunities of learning the existence of such 

 glassy cells. The stinging hair of the nettle is simply 

 a long-pointed cell, the walls of which, especially at the 

 top, are as brittle as glass, because they are full of silica ; 

 this is why they pierce the skin so easily, break in the 

 wound, and inject their poisonous sap. Large quantities 

 of silica are contained in the straw of cereals and in the 

 stem of the horse-tails. The latter are so hard that 

 carpenters use them for polishing wood. 



Silica, then, is found very generally among plants, and 

 we might suppose it to be indispensable to the plant. An 

 idea has grown up to the effect that it not only adds 

 hardness to the external tissue of cereals, but even gives 

 solidity and firmness to the whole body of the straw ; it 

 has been supposed that by increasing the supply of silica 

 in our cultivated cereals, these would be less readily laid 

 by wind and heavy rain, which do so much damage to 

 corn. But direct experiment has put an end to all these 



