86 PHOTOSYNTHESIS 



its carbon dioxide from the surrounding water in which the gas is 

 dissolved. The water is in equilibrium with oxygen and nitrogen of 

 the atmosphere, the solubility of these two gases in water being relatively 

 low as compared with carbon dioxide. The carbon dioxide reaches the 

 centers of photosynthetic activity, the chloroplasts, by diffusion. Here, 

 through photosynthesis, the carbon dioxide is reduced, and we may 

 assume with a fair degree of safety, an equal volume of oxygen is 

 formed. This oxygen can migrate either an exceeding small distance to 

 the intercellular spaces, or in the other direction, out of the plant into 

 the surrounding water. What actually happens is that the oxygen escapes 

 from the plant through the intercellular spaces. If it were simply a 

 matter of dififerences in diffusion coefificients the oxygen would never 

 appear as bubbles of the gas. Kniep erroneously assumes that carbon 

 dioxide has a very much higher diffusion coefficient than oxygen. 

 Actually the reverse is the case and, strictly speaking, the diffusion 

 coefficient is independent of solubility. However, carbon dioxide is much 

 more soluble in water than oxygen and the water is already saturated 

 with the latter gas at ordinary pressures and temiDeratures. The oxygen 

 which is formed in photosynthesis produces a slight increase in pressure, 

 partially dissolves and diffuses out, but mainly escapes into the inter- 

 cellular spaces. Assuming the water is under atmospheric pressure, the 

 gas will escai^e from the intercellular spaces when the pressure therein 

 has attained a value slightly higher than atmospheric pressure and suffi- 

 cient to overcome the resistance of the capillary surfaces in the inter- 

 cellular canals. As this pressure increases more oxygen is dissolved 

 in the water bounding the intercellular sj^aces and escapes by diffusion. 

 Therefore, all of the oxygen produced in photosynthesis does not escape 

 as a gas, although on account of the low solubility of oxygen in water 

 the quantity removed by diffusion is very small compared to the gaseous 

 emission. 



The higher the rate of oxygen formation the faster the gas will 

 escape from the intercellular spaces. But the water contains dissolved, 

 not only oxygen, but also nitrogen and carbon dioxide. Therefore these 

 gases are also present in the intercellular spaces. The bubbles which 

 escape from these spaces contain these gases also. As has been stated, 

 the higher the rate of oxygen formation, the faster the gas will escape, 

 sweeping out with it the nitrogen and carbon dioxide. As the pressure 

 of these latter gases is reduced in the intercellular spaces the equilibrium 

 with the water bounding the spaces is disturbed and some nitrogen and 

 carbon dioxide pass from the dissolved state into the gaseous form. 

 When the rate of photosynthesis is high and the escaping gas stream 

 correspondingly high, the oxygen sweeps out some of the nitrogen and 

 carbon dioxide faster than it can be replaced. Thus, per unit time, with 

 a high rate of oxygen formation, much nitrogen is carried out. This 

 represents the condition in which the oxygen-content of the escaping gas 



