CONSTITUENTS OF PLANTS. 103 



potassium, nitre, or nitrate of strontia ; they will 

 enter into the different parts of the plant, just as 

 the coloured juice mentioned above, and will be 

 found in its ashes if it should be burnt at this 

 period. Their presence is merely accidental ; but 

 no conclusion can be hence deduced against the 

 necessity of the presence of other bases in plants. 

 The experiments of Macaire-Princep have shown 

 that plants made to vegetate with their roots in a 

 weak solution of acetate of lead, and then in rain- 

 water, yield to the latter all the salt of lead which 

 they had previously absorbed. They return, there- 

 fore, to the soil all matters which are unnecessary 

 to their existence. Again, when a plant, freely 

 exposed to the atmosphere, rain, and sunshine, is 

 sprinkled with a solution of nitrate of abrontian, the 

 salt is absorbed, but it is again separated by the 

 roots and removed further from them by every 

 shower of rain, which moistens the soil, so that at 

 last not a trace of it is to be found in the plant. 



Let us consider the composition of the ashes of 

 two fir-trees as analysed by an acute and most 

 accurate chemist. One of these grew in Norway 

 on a soil, the constituents of which never changed, 

 but to which soluble salts, and particularly common 

 salt, were conveyed in great quantity by rain-water. 

 How did it happen that its ashes contained no 

 appreciable trace of salt, although we are certain 

 that its roots must have absorbed it after every 

 shower ? 



We can explain the absence of salt in this case 



