EXCREMENTS OF PLANTS. 73 



merely accidental ; but this does not furnish ground for any con- 

 clusion against the necessity of the presence of other bases in 

 plants. The experiments of Maeaire-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, 

 therefore, to the soil all matters unnecessary to their existence. 

 Again, when a plant, freely exposed to the atmosphere, rain and 

 sunshine, is sprinkled with a solution of nitrate of strontia, the 

 salt is absorbed, but it is again separated by the roots and 

 removed further from them by every shower of rain that falls 

 upon the soil, so that at last not a trace of it is to be found in the 

 plant. (Daubeny.) Let us consider the composition of the 

 ashes of the two fir-trees above mentioned as analysed by an 

 acute and most accurate chemist. One of these grew in Nor- 

 way, on a soil of invariable composition, but to which soluble 

 salts, and particularly common salt, were conveyed in great 

 quantity by rain-water. How did it happen that its ashes con- 

 tained 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 by means 

 of the direct and positive observations referred to, which have 

 shown that plants have the power of returning to the soil all 

 substances unnecessary to their existence ; and the conclusion to 

 which all the foregoing facts lead us, when their real value and 

 bearing are apprehended, is that the alkaline bases existing in 

 the ashes of plants must be necessary to their growth, since if 

 this were not the case they would not be retained. 



The perfect development of a plant, according to this view, is 

 dependent on the presence of alkalies or of alkaline earths ; for 

 when these substances are totally wanting its growth will be ar- 

 rested, and when they are only deficient it must be impeded. 



In order to apply these remarks, let us compare two kinds of 

 trees, the wood of which contains unequal quantities of alkaline 

 bases, and we shall find that one of these may grow luxuriantly 

 in several soils upon which the other is scarcely able to vegetate. « 

 For example, 10,000 parts of oak-wood yield 250 parts of ashes, 



