Vegetable Assimilation and Respiration. 



309 



(B) Method of Diminishing Errors due to Gaseous Diffusion in the Water round 



the Plant. 



As the gaseous exchange of the assimilating plant takes place in the centre 

 of a mass of water, its supply of CO2, when the water is not in movement, 

 depends upon diffusion from all around the plant. This factor is particularly 

 important in laboratory experiments with vessels of very still water, and but 

 slight temperature-convection currents. Certain effects due to this cause 

 have not always been rightly understood. Similarly with the oxygen pro- 

 duced : this may leave the plant by the path of diffusion into the water as 

 well as by escape through the intercellular spaces. Both these matters have 

 been sources of illusion in early experiments. 



Ba. Aberrations due to Deficient Oxygen in Solution : the " Initial Oxygen 

 Diffusion Effect." — The less oxygen there is in solution in the water the greater 

 must be the proportion of photosynthetic oxygen which diffuses away into 

 the water and fails to record itself as bubbles. Most observers have used for 

 their work water in which the oxygen in solution is no greater than that due 

 to equilibrium with the atmosphere, so that the water against the plant only 

 contains initially 21 per cent, of the volume of oxygen it can hold in solution 

 in equilibrium with pure oxygen. In such a medium, when the plant is 

 first illuminated, the bubbling rate is very slow, but steadily increases, more 

 and more of the oxygen escaping as bubbles. The increase is at first rapid 

 and then slows up smoothly towards an approximately constant rate, reached 

 when only a little is passing away by diffusion. Fig. 2 gives an excellent 

 illustration of the effect. These data are taken from Angelstein (p. 97). 

 The nearly level rate that is obtained in due time is not absolutely stable, 

 but depends on a steady diffusion gradient being maintained. Any small 

 mechanical shake is liable to disturb it. Stirring brings the bubble rate 

 down instantaneously and then it slowly climbs up again. This phenomenon 

 has necessarily been encountered by all workers. Some have discarded these 

 initial periods altogether, some recorded them as disturbing factors without 

 explanation, and others have interpreted them correctly, but no one seems to 

 have attempted to eliminate them for their research work, systematically, by 

 charging the solutions to be used with a larger amount of oxygen. In working 

 on the effect of drugs and other solutes on bubbling plants time is precious 

 and the initial period cannot well be spared. 



In the work on effects of acids and bicarbonates which follows, water was 

 used whiish was charged with oxygen by prolonged violent shaking in bottles 

 with an atmosphere of pure oxygen from a cylinder. The charging process is 

 very slow in its later stages. Such water was used for diluting the standard 



2 A 2 



