THE AEEATION OF PLANTS 115 



containing a good deal of the former gas is in contact with 

 wet cell-walls, the result of the active absorption will be to 

 set up a stronger current to that spot than would be the 

 case if oxygen replaced it. Any cessation in the absorption 

 of carbon dioxide by the green cells owing to diminution of 

 light must be attended by a certain variation in the gaseous 

 stream. The ways in which alterations in the absorption of 

 oxygen will affect the currents will also be readily apparent. 

 During bright sunlight, when both processes are proceeding 

 in the same and in different parts of the plant, local 

 positive pressures of either oxygen or carbon dioxide may 

 occur, and it is evident that the direction of the gaseous 

 currents will vary very much in consequence. 



The structure of the plant has a certain influence on 

 the composition of its internal atmosphere. The epidermis 

 of most terrestrial plants is strongly cuticularised, while 

 there is but little cuticle to aquatics. The entry of gases 

 into the latter is accordingly easier than it is into the 

 former, penetration into which must take place through 

 the stomata. Moreover, the larger reservoirs in the 

 interior of aquatics serve to equalise the composition of 

 the internal atmosphere, and to cause it to resemble more 

 closely that of ordinary air. 



Such plants again as contain no green colouring mat- 

 ter for example, the bulkier Fungi, which require provision 

 for the supply of air to their interior have only the one 

 metabolic process in which the interchange of oxygen and 

 carbon dioxide is involved, the former being absorbed, and 

 the latter exhaled. To a corresponding extent, therefore, 

 the gaseous currents are simplified, though even' in these 

 plants the direction and the amount are never constant for 

 long together, the metabolism continually varying. 



In another important respect the internal air of plants 

 differs from that of the atmosphere. It is always charged 

 with aqueous vapour, frequently even to the saturation 

 point, as we have seen in connection with the process of 

 transpiration. 



