38 PHYSIOLOGY OF NUTRITION. 



mometer, in a beaker of water containing Carbon dioxide in 

 solution. The temperature of the water may be, say, 12 C- We 

 count the bubbles liberated in a certain time, e.g., 1, 3, or 5 

 minutes, in access of light. We then warm the water in the 

 beaker, without removing the plants from it, to a temperature of 

 about 24 C. It will be found that the number of bubbles pro- 

 duced by the Elodea or Hippuris is now considerably greater than 

 at the lower temperature. Care must of course be taken that the 

 research material in the two experiments is exposed to light as 

 nearly as possible constant in intensity ; it is best therefore to 

 make the experiments when the sky is quite cloudless. The 

 optimum temperature for liberation of Oxygen in Elodea and 

 Hippuris is not yet accurately determined, but may be taken to 

 be about 32 C. At temperatures beyond this the liberation of 

 Oxygen proceeds more slowly again. 2 



We expose some Elodea twigs to the influence of the light in 

 Carbon dioxide-containing spring water which has previously been 

 shaken with chloroform. The liberation of Oxygen continues for 

 a remarkably long time (in any experiments for more than a 

 quarter of an hour) ; finally it ceases entirely, owing to the poison- 

 ous action of the chloroform. 3 



To prove that chlorophyll-free plant structures are unable to 

 assimilate, all that is necessary is to expose to the light, in Carbon 

 dioxide- containing spring water, pieces of root for example. 

 Liberation of Oxygen does not take place. 



A very interesting method of proving the assimilatory activity 

 of green cells has been introduced by Engelmann. 4 From a pure 

 culture of Bacterium Termo, prepared in the manner to be de- 

 scribed later, we take swarming individuals, place them on the 

 slide in a drop of water, lay on the cover glass, and seal its edges 

 with vaseline. With strong magnification we make out that the 

 bacteria, being highly dependent upon the presence of Oxygen, 

 soon come to rest; they only continue in movement in the neigh- 

 bourhood of air-bubbles occurring in the preparation, and there 

 also they ultimately come to rest. If however we introduce into 

 the preparation with the swarmers an algal filament, the move- 

 ment of the bacteria, as long as the alga is illuminated, continues 

 without interruption. The Oxygen produced by the chlorophyll 

 bodies acts as a stimulus, and occasions the movement, as also the 

 direction of movement, of the bacteria. They collect in the 

 neighbourhood of the Oxygen-distributing alga, and if, for example, 



