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. 
