RELATIONS OF PLANTS TO GASES 189 



may be mentioned : lenticels which furnish connection between 

 the cortex of stems and roots, and the outer air through the cork, 

 and stomata connecting green tissues of leaves and other organs 

 with the atmosphere. The vessels and tracheids which usually 

 contain gases in mature plants, have no direct connection with 

 the air in the intercellular spaces. Any exchange with them 

 must take place by osmose through one or more membranes, since 

 it is impossible to force gas through a membrane by pressure as a 

 liquid might be transmitted. The withdrawal of water from cells 

 by drying out, or diffusion, may reduce the pressure below that 

 of the atmosphere, and in some instance results in an almost 

 perfect vacuum, since the air may not penetrate the wall except 

 by diffusion, unless actual openings are present. Then again it 

 is to be said that carbon dioxide diffuses much more rapidly than 

 oxygen, so that cells in which rapid respiration is in process 

 show a reduced pressure. It is to these causes that the negative 

 pressure of the shoots and branches of large woody plants is 

 principally due. The unequal diffusibility of the atmospheric 

 gases also varies the composition of the air enclosed in the closed 

 vessels of a plant. 



The gases in the intercellular spaces may diffuse with great 

 rapidity into the thin-walled cells with which they are in contact. 

 The gaseous exchange between the plant and the atmosphere is 

 regulated to some extent however, by variations in the width of 

 the epidermal openings, i. e., the stomata and lenticels. The 

 stomata are in general under the control of mechanisms by which 

 they may be opened or closed in a few seconds, while the lenti- 

 cels undergo seasonal changes. The diffusion of the gases of 

 the air through the stomata and lenticels is not exactly similar 

 to the rate through capillary openings. Thus it has been found 

 that the flow of gas through a tube is proportional to the sec- 

 tional area of the column of gas. It is known however, that 

 if the flow is partially obstructed at any point by a thin septum 

 pierced with a circular aperture, the rate of flow across unit 

 area of aperture is greater than it would be across an equal area 



