CHAPTER IX. 



THE AERATING OR VENTILATING SYSTEM. 



I. GENERAL CONSIDERATIONS. 



All plants carry on a more or less active gaseous interchange with the 

 surrounding atmosphere. During respiration, oxygen is used up and 

 carbon dioxide excreted. The process of photosynthesis, which is 

 characteristic of the green parts of the plant-body, involves the opposite 

 type of interchange, carbon dioxide being absorbed and oxygen 

 evolved. In certain cases other gases, such as hydrogen or sulphuretted 

 hydrogen, are produced as the result of special katabolic changes. 

 Large quantities of water-vapour escape in consequence of the trans- 

 piration which goes on in all the aerial organs. Many plants, finally, 

 give off vapours consisting of " ethereal oils " and other volatile organic 

 compounds. 



Other things being equal, the activity of any of these forms of 

 gaseous interchange depends upon the extent of surface through which 

 gaseous diffusion can take place. In the case of microscopic unicellular 

 organisms, the ratio of superficial area to volume is so large that 

 special arrangements for increasing the free surface are entirely un- 

 necessary. The larger a plant is, however, the greater does its need 

 become for increase of superficial area. This demand can be met up 

 to a point by more or less extensive ramification of the plant-body ; the 

 additional free surface acquired in this way, may by itself enable a plant 

 of moderate size to carry on and regulate all its various forms of gaseous 

 interchange in a perfectly satisfactory manner. Among highly organised 

 [land-] plants, however, the external surface alone can never be 

 sufficient for the purpose, even when the plant-body is divided up to 

 the utmost possible extent. In these circumstances, the plant resorts 

 to the formation of an internal labyrinth of air-containing cavities and 

 channels, which stand in the same relation to the adjoining tissues as 

 does the outer atmosphere to the superficial cells. The ventilating 



