SECOND REPLY. 167 



point out, in every individual case, what import this or that 

 particular substance possesses in the life of the plant. It 

 suffices that we know that these substances are indis- 

 pensible to the healthy growth of certain plants. 



Novel and strange as the assertion may at present appear 

 to many, that the insignificant quantity of ash in a plant 

 deserves our chief attention in its life, it will readily be 

 allowed and become familiar, so long as and because this 

 circumstance is always regarded as a secondary consideration, 

 if also in its way a necessary one. But the matter assumes 

 quite another aspect when, acquainted with the fundamental 

 principles and the course which science must and will take in 

 the coming time, we now anticipate the final results, for the 

 complete establishment of which we have perhaps to labour 

 for another century. Thus then will run our aphorism : 

 The whole wealth and the whole manifoldness of terrestrial 

 vegetation, its whole variety, as well when we compare zones 

 of longitude and latitude as wild nature with cultivated 

 lands, are exclusively dependant on the variety of inorganic 

 constituents which the plant takes up from the soil. When 

 we look to the wild vegetation of our own latitudes, we find 

 two principal classes of soil : one a peat or bog soil, which 

 consists almost wholly of humus, therefore of decomposed 

 organic matter, the other of calcareous, sandy, or argil- 

 laceous soils, in which the inorganic constituents prevail in 

 so great a degree, that the humus, in the blackest soils, does 

 not amount to more than 10 per cent, at most, and even 

 in the most fertile, and those clothed with the richest vege- 

 tation, often scarcely forms 4 per cent. And that peat or 

 bog soil, so rich in humus, can only afford sustenance to 

 300 of the 5,000 flowering plants growing in central Europe ; 

 and there are not perhaps fifty plants, therefore not one per 

 cent, of which the actual conditions of healthy growth are 



