THE AERATION 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. 
