THE MOLECULAR FORCES IN PLANTS. 



181 



under such conditions air is forced into the intercellular spaces, 

 which, as recent researches have shown, are not entirely absent 

 even in the wood, though they are there certainly very narrow. 5 



It is even possible moreover, at any rate in some plants, that 

 air can at high pressure pass in small quantities from the inter- 

 cellular spaces through the membranes of the vessels (gas nitra- 

 tion). 



It is also instructive to investigate the phenomena under con- 

 sideration with the help of the apparatus depicted in Fig. 65, 



FIG. 64. Poroscope. 



FIG. tf 5. Apparatus for 

 investigating the movement 

 of gases in the plant. (After 

 Pfeffer.) 



using leaves of Heracleum, ^Bgopodium, etc., or shoots of various 

 plants (e.g. of Prunus Padus). The glass cylinder g is closed with 

 a cork, in which the leaf-stalk or shoot-axis has been cemented 

 air-tight. The cylinder/, which contains water, may fit on the- 

 same cork. The cut surface of the object lies jnst below the 

 surface of the water, so that it can be observed through a suitably 

 mounted microscope magnifying twenty or forty times. If now 

 mercury is poured into the tube /, so as to compress the air in g, 

 which it is best to keep moist by means of a layer of water on 

 the mercury, the air enters the stomata and escapes at the cut 

 surface, as the microscope shows, only from the cortex and pith. 

 If we increase the pressure, small quantities of air may also 



