THE AEEATION 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. 



