THE AERATION OF PLANTS 117 
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 rela- 
tion 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 atmo- 
sphere. The true gaseous interchanges which subserve the 
life of the protoplasts, and hence of the plant, take place 
not at the stomatal orifices, but completely throughout the 
interior of the substance of the plant. 
