CHEMICAL BASIS OF PLANT FORMS 239 



the especial absorptive function of leaves is the absorption of 

 gases, as has been already explained. 



The subject of the ash-constituents of plants is a very impor- 

 tant one in this connection. The essential mineral constituents 

 of plants have already been mentioned; silicon, fluorine, manga- 

 nese, sodium, lithium, rubidium, caesium, barium, aluminium, 

 zinc, copper, titanium, iodine, and bromine have also been 

 found among the ash ingredients of certain plants. 



The method of absorption of soluble mineral salts has already 

 been described. A solution of insoluble salts is brought about 

 in a different way. A soil rich in organic matter is always 

 charged with carbon- dioxide, and this gas is also given off by 

 the roots of living plants. Water containing this gas is able to 

 dissolve calcium carbonate and some silicates that are insolu- 

 ble in pure water. The presence of certain soluble salts in the 

 soil brings about a decomposition and renders the insoluble salts 

 more readily soluble. Finally, the insoluble salts are brought 

 into solution by means of the acid sap which saturates the cell 

 wall of the root hair. This acid is not carbonic acid, for its red- 

 dening of litmus paper is permanent. 



It has been shown by experiment that the chemical elements 

 are not universally absorbed by roots in their combinations in 

 the soil. 



The wide differences in the composition of the ashes of 

 plants show that each plant is endowed with a specific absorb- 

 ent capacity. It is upon this fact that the "rotation of crops" 

 in farming depends. A gramineous plant 1 is able to withdraw 

 relatively larger quantities of silica from the soil than a legu- 

 minous plant. The latter can only do so to a very slight extent. 



The absorbent capacities of nearly allied species are very dif- 

 ferent; again, individuals of the same species yield different ash 

 compositions, depending upon their vigor; and the absorbent 

 capacity of the plant varies at different periods of its life. It has 

 been stated that " similar kinds of plants, and especially the 

 same parts of similar plants, exhibit a close general agreement 

 in the composition of their ashes, while plants which are unlike 

 in their botanical characters are also unlike in the proportions 



1 Wolff, Aschenanalysen, 1871. 



