THE FOOD OF PLANTS. 137 



pound of potassium which is most advantageous to the plant. 

 Farsky's observations, however, seem to shew that chlorine 

 is itself of importance. 



Silicon. Silicon, as already pointed out in the fourth lec- 

 ture (p. 54), is absorbed in the form of soluble silicates or 

 possibly as soluble silicic acid. It principally occurs in 

 plants in the cell-wall, but Lange appears to have found it as 

 silicic acid dissolved in the cell-sap in Eqidsetum hiemale, and 

 Pfitzer has described certain cells in the pseudobulbs of epi- 

 phytic Orchids which contain each a plate of silica. 



Inasmuch as silica is always present in the ash, and fre- 

 quently in very large quantity (see p. 21), it was thought that 

 silicon must be essential to the nutrition of plants. Sachs found, 

 however, that a Maize-plant will grow in a solution containing 

 no silicon, though the ash of such a plant contains but 07 per 

 cent, of silica, whereas that of a plant grown under ordinary 

 conditions from 18 to 23 per cent, and his results have been 

 confirmed by many observers. Wolff has, however, found 

 by means of water-cultures, that in the case of Oats, the 

 number of perfect seeds formed is greater when the plant 

 is abundantly supplied with silica. 



Still it is possible to imagine that even if silicon does not 

 play any important part in the metabolism of the plant, it 

 may be of use to it in giving firmness and rigidity to its tis- 

 sues. It was suggested by Humphry Davy, and the view 

 has found many supporters, that the cause of the "laying" 

 of wheat and other cereals might be an insufficient supply of 

 silica. On investigation this has been found not to be the 

 case. The structure of the haulm is not affected by the supply 

 of silica. The real cause of "laying" has been found to be the 

 imperfect development of the tissues due to an inadequate ex- 

 posure of light, a point which will be considered hereafter. 



Inasmuch as all the chemical elements are present in one 

 place or another in the soil, it will not be surprising to find 

 that they have nearly all been detected at different times in 

 the ashes of plants ; there is no need therefore to enumerate 

 further the ash-constituents. 



In the next lecture we shall consider the changes which 



