CHAPTER V 



THE MECHANISM OF GASEOUS EXCHANGE 



SECTION 29. General. 



IT is well known that gaseous substances play a most important part 

 in metabolism : thus, in aerobic plants a sufficient supply of oxygen must 

 be ensured, and the carbon dioxide produced by respiration must be removed. 

 Green plants, when illuminated, decompose large quantities of carbonic acid 

 gas and produce free oxygen, although of the former gas traces only are 

 present in the atmosphere. Many plants, especially ferment-organisms, 

 produce besides carbon dioxide, other gases, such as hydrogen, sulphuretted 

 hydrogen, and carburetted hydrogen or marsh gas. Neutral gases, such as 

 hydrogen or nitrogen, also penetrate the plant by means of its air spaces, 

 and permeate the living cells as well, even when not utilized in metabolism. 



In a few cases gas-vacuoles may be present (cf. Sect. 22), but otherwise 

 turgid cells always contain gases in dissolved form, the solution of the 

 latter being a necessary consequence for their passage through the cell-wall 

 saturated with imbibed water and penetration to the interior of the cell. 

 The gaseous molecules are absorbed by the layer of imbibition-water against 

 which they collide, and diosmose through to the interior in a dissolved con- 

 dition. Any internal accumulation necessarily leads to an exosmosis of the 

 gas produced, which is given off to the external air in gaseous form by the 

 over-saturated imbibition water on the free surface of the cell. 



The mode of absorption is essentially similar when the cell is 

 surrounded by a layer of water, which absorbs fresh supplies of oxygen 

 or carbonic acid from the air, as the cells which consume these gases 

 extract them from the fluid in which they lie. The outward passage of 

 internally produced gases does not usually lead to any formation of air 

 bubbles upon the free surface of the cell, which only occurs when the rate 

 of excretion is so rapid that the water becomes locally over-saturated. 

 Owing to the high internal osmotic pressure, super-saturation is hardly 

 possible within the cell, and hence certain special factors must be responsible 

 for the formation and maintenance of gas-vacuoles. 



In a turgid cell the entry or exit of gas differs from that of liquid 



