234 PHOTOSYNTHESIS 



the rate at which it is removed by the passage out with the oxygen. 

 During very active photosynthesis the absolute amount of nitrogen emitted 

 is greater than during a low rate of photosynthesis, although the per 

 cent of nitrogen liberated under the former conditions is lower than during 

 the latter. 



Another factor which must receive consideration in the discussion of 

 this method is the solution of oxygen from the plant to the surrounding 

 water. Kniep has shown experimentally that this is not a negligible 

 factor. According to Henry's law the solubility of a gas in a given 

 volume of water is proportional to the pressure of that gas. Assum- 

 ing then that the pressure of the gas in the intercellular system is equal 

 to that of the atmosphere, even in weak photosynthesis there would be 

 the conditions for oxygen going into solution, for solution depends upon 

 the partial pressure of the intercellular gas and this for oxygen during 

 photosynthesis exceeds that of the atmosphere which is in equilibrium 

 with the water. As Kniep furthermore showed, water which has been 

 covered with mineral oil and saturated by running a stream of oxygen 

 through it increases in oxygen content when Elodea is permitted to carry 

 on photosynthetic activity. A similar increase in Oo-content is obtained 

 when water which is in equilibrium with the atmosphere is placed over 

 photosynthetically active plants. 



Movement of the water surrounding the plants has a decided effect 

 on the rate at which the bubbles are given off. The contradictory re- 

 sults of Darwin and Pertz ^^ and of Nathanson ^^ have been explained 

 by Kniep in the paper referred to above. The difficulty lies primarily in 

 the interfering of the physical stream of diffusion through the plant. 



The rate of bubble emission is decreased in moving water when this 

 is not saturated with the atmospheric gases. The oxygen which is formed 

 in photosynthesis escapes partly as bubbles and partly it is dissolved 

 in the water surrounding the plant. Thus in quiet water an area about 

 the plant becomes saturated with oxygen and more oxygen escapes as 

 bubbles. If now the water about the plant is in motion this saturated 

 area is removed, a larger proportion of the oxygen is dissolved and 

 less gas escapes as bubbles. 



The rate of bubble emission is increased in moving water when this 

 is supersaturated with the gases of the atmosphere. Under these cir- 

 cumstances there is a pure physical stream of diffusion through the plant, 

 long ago described by Van Tieghem, and already referred to. This 

 stream is augmented either by movement of the water or by slight jarring 

 of the plant. 



There is another important phenomenon of bubble emission which has 

 been described by Kniep. When the bubble stream from a plant is in- 

 terrupted by withdrawing the light, and the plant is then reilluminated, 



"Darwin, F., and Pertz, Proc. Cambridge Philos. Soc, 9, 11-76 (1896). 

 "Nathanson, A., Ber. d. Verh. d. Kgl. Saechs. Ges. d. Wiss., 59, 711 (1907). 



