202 Elementary Biology. 



undergoes, that is to say, the preliminary changes of ana- 

 bolism. The supply outside the root is greater than that 

 inside, hence more is absorbed, anabolism constantly re- 

 moving what osmosis is constantly supplying. We are here, 

 of course, considering only such substances as are soluble in 

 water, and which can penetrate through cell- wall and proto- 

 plasm into the cell-sap. What of those substances which 

 are insoluble in water? How, for instance, can calcic 

 carbonate be absorbed by the root when that substance is 

 insoluble in water? It has been already shown that all 

 living cells, in virtue of the katabolism which they are 

 undergoing, are constantly giving off, or respiring, carbonic 

 acid (p. 40). This carbonic acid, in the case of the cells of 

 the roots, does not readily escape, but, becoming dissolved 

 in the water surrounding the root cells, acts on the calcic 

 carbonate, changing it into the bicarbonate, which is soluble 

 in water. In this form calcium is taken into- the plant. No 

 doubt many organic acids are also employed by the plant in 

 the work of rendering insoluble substances soluble. 



The amount, then, of the various compounds in the soil 

 which are found in the individual plant depends on the 

 specific absorbent capacity of the plant, and on the compo- 

 sition of the soil. Several examples of the composition of 

 the ash of plants have been already given at p. 195. 



Leaves, stems, and even flowers are able under certain 

 circumstances to absorb water and salts in solution, a fact 

 familiarly known to all lovers of floral decoration. 



(b.) Ab:orption of gases. Since, as mentioned above 

 (p. 191), free nitrogen is not used by the plant, we need not 

 consider its absorption at all. Indeed, the plant is always 

 bathed in free nitrogen, though no direct assimilation of the 

 gas takes place. The two gases whose absorption we have 

 specially to study are oxygen and carbonic acid. 



Absorption of oxygen. Oxygen is absorbed by the plant 

 for two purposes, to assist in the formation of organic com- 

 pounds and also to oxidise these same compounds in order 



