64 QUARTERLY JOURNAL. 



iron, manganese, chlorine, iodine, sulphur and phosphorus. These 

 do not exist in the plant in their simple form, but variously com- 

 bined with each other and with another class of substances to 

 be mentioned hereafter. Till a very recent date they were re- 

 garded as not essential to the vegetable structure, being considered 

 as being only accidentally present, from the fact of the plant 

 growing in the earth. They are now however, regarded as not 

 only important, but essential in the economy of vegetation, and it 

 is the opinion of Liebig, that the presence of these substances ex- 

 ercises the whole control in perfecting the plant. If they were 

 not necessary to the healthy growth of the plant, or were only 

 accidentally present, we should expect that sometimes one would 

 be found in plants of the same species, and sometimes another in 

 greater proportion than at other times, or that plants of whatever 

 kind growing in the same soil would contain the same quantities. 

 But this is not the case. The variation is very slight in plants of 

 the same kmd, and those proportions may be considered as uni- 

 form. Nor do plants of different species, nor even those of dif- 

 erent varieties of the same species, growing in the same soil, con- 

 tain the same quantities. In the tables quoted above, a remarka- 

 ble difference appears in the quantity of ash left by various plants, 

 and the difference is as remarkable in the constituents of this ash. 

 Thus, the straw of wheat gives more than three per cent of ash, 

 in which the principal mineral ingredients are as in the following 

 table compared with those of oats, yielding nearly six per cent, 

 and beans more than three per cent. The quantity is what is 

 found in 1,000 lbs. of straw. 



Wheat, Oats, Bean, 



lbs. lbs. lbs. 



Potash, 0.20 8.70 16.56 



Soda, 0.29 0.01 0.50 



Lime, 2.50 1.52 6.20 



Magnesia, 0.32 0.22 2.09 



Sulphuric acid, 0.37 0.79 0.34 



Phosphoric acid, 1.70 0.12 2.26 



Silica, 28.70 45.88 2.20 



A remarkable difference will be observed in the proportions of 

 these substances in the different straws, and especially in the pot- 

 ash and silica. While the wheat straw contains potash 0.20, 



