ABSORPTION OF GASES. 73 



The absorption of gases is effected then, principally at 

 least, by the superficial cells of the leaves. Now as to the 

 mode of absorption. We may consider here, once for all, 

 the mode in which gases are absorbed by the cells of plants, 

 for it is the same in all cases. Gases, like solid substances, 

 can only be absorbed in solution. They may be brought to 

 the surface of the cell-wall already dissolved in water, as in 

 the case of submerged plants, or they may be dissolved from 

 the atmosphere by the sap which saturates the cell-wall, 

 as in the case of land-plants. In either case they reach the 

 interior of the cell in solution. When a gas has been taken 

 up at the surface it diffuses throughout the cell-sap, and thus 

 fresh quantities can be taken up from without until the limit 

 of solubility is reached, when absorption ceases. If, however, 

 the metabolism of the cell changes the chemical condition 

 of a gas, if it causes its decomposition or causes it to enter 

 into new combinations as it is absorbed, then its absorption 

 will be continuous. On comparing these statements with 

 what was said as to the absorption of substances by the roots, 

 we find that the conditions of absorption are essentially the 

 same in the two cases. 



There exists another important similarity between the 

 absorption of gases and the absorption of substances in solu- 

 tion, namely this, that just as the root can only absorb a 

 solution below a certain degree of concentration, so the leaf 

 can only absorb a gas below a certain degree of pressure. 

 For instance, the pressure of the carbon dioxide in the air 

 is very slight ; it was first observed by Percival that an 

 increase in the quantity of carbon dioxide in the air is 

 favourable to the growth of the plant ; de Saussure found 

 that a considerable increase is prejudicial, and Godlewski, 

 by his more detailed investigations, shewed that the optimum 

 proportion is from 8 10 per cent, that is that carbon 

 dioxide is most readily absorbed by the plant when its 

 pressure is about 200 times greater than in ordinary air. 

 Boussingault found that when leaves are exposed to sunlight 

 in an atmosphere of pure carbon dioxide at the ordinary 

 pressure they cannot decompose it, but if the carbon dioxide 



