ABSORPTION AND TRANSMISSION OF SOLUTES 117 



During the hours of sunlight, when the process of pho- 

 tosynthesis is going on, carbon dioxid is being combined 

 with water to produce carbohydrates within the chlorophyll- 

 bearing cells. Thus, at this time the diffusion tension of 

 this gas in the solutions of these cells is much lower than it 

 is in the outer air and in the air chambers. Therefore, 

 there must be a constant diffusion stream of carbon dioxid 

 moving through the stomata into the air chambers, going 

 into solution in the imbibed water of the moist cell walls 

 wherever it touches them, and diffusing as a solute through 

 the tissues of the plant to the places of low diffusion tension. 



The oxygen which is given off in photosynthesis finds 

 its way to the outer air in the same manner as that by which 

 the carbon dioxid enters. The oxygen tension becomes 

 higher in the green cells than in the outer air, and a diffu- 

 sion stream of this gas is at once set up in the direction of 

 the air chamber, where it goes out of solution and then dif- 

 fuses as a gas through the stomata into the outer air. Dur- 

 ing the night when photosynthesis has ceased this oxygen 

 stream slackens and stops, as does also the incoming stream 

 of carbon dioxid. What oxygen is used in respiration at 

 this time enters from the outer air, and the carbon dioxid 

 produced by this process finds its way out in a manner 

 exactly similar to that in which the other gas escapes during 

 the day. 



Epidermal tissues of leaves and stems are also imbibed 

 with water, but the amount of water which they can hold is 

 comparatively small on account of the fact that the external 

 walls are heavily impregnated with waxy substances. Hence, 

 as would be expected, a small amount of gaseous exchange 

 between the atmosphere and the plant takes place directly 

 through these membranes. 1 



i F. F. BLACKMAN, "Experimental Researches," etc.: II, "On the Paths of 

 Gaseous Exchange between Aerial Leaves and the Atmosphere," Phil. Trans. Roy. 

 Soc., London, B., Vol. CLXXXVI (1895), pp. 503-62. 



