THE AEEATION 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 Myriopliyllum, 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. 



