THE AERATION OF PLANTS 123 
cases of woody stems, but it can be seen also in plants in 
which the development of wood is only very slight, having 
been observed in some cases in the elements of the central 
cylinder of some of the stouter Mosses. 
To demonstrate the existence of the negative pressure 
in the vessels of the stem, a young plant should be removed 
from the soil and allowed to become flaccid. The stem 
should then be partially immersed in mercury and cut 
across below the surface of the latter. The mercury will 
immediately rise to some distance in the vessels, being 
drawn up by the suction exerted by the negative pressure 
therein. 
An actual positive pressure can under certain conditions 
be observed in the intercellular air-reservoirs of particular 
plants. This can be shown by cutting the stems of sub- 
merged plants such as Myriophyllum, when, if they are 
brightly illuminated, bubbles of gas may be seen to emerge 
from the cut end. This positive pressure appears to be 
due to a considerable production of oxygen by the green 
parts of the plant under the conditions of illumination, as 
it varies with the intensity of the latter, and ceases entirely 
in darkness. 
It is well that we should lay some stress upon the nature 
of the relation which the stomata show to the processes of 
gaseous interchange. Though they are the chief means of 
the entry of gases into and their exhalation from the plant, 
it is misleading to speak of them as the organs of such gaseous 
interchange. The actual processes of interchange take 
place between the protoplasts and the air of the intercellular 
reservoirs, so that the latter are the special organs devoted 
to such functions. The stomata and the lenticels are merely 
the openings by which the air of these internal formations 
communicates with the outer atmosphere. The true 
gaseous interchanges which subserve the life of the proto- 
plasts, and hence of the plant, take place not at the stomatal 
orifices, but completely throughout the interior of the 
substance of the plant. 
