36 PHYSIOLOGY OF NUTRITION. 



glass filled with water holding Carbon dioxide in solution. Over 

 the end of the tube of the funnel, which is below the surface of 

 the water, is inverted a test-tube filled with water, and the whole 

 is exposed to direct sunlight. The Oxygen, or, to speak more 

 accurately, the air rich in Oxygen collects, as in the foregoing 

 experiment, in the test-tube. Material for these experiments will 

 be at our disposal for a considerable part of the winter if in 

 autumn we put vigorous plants of Elodea into a large vessel of 

 water, and place this at the window in a warm room, frequently 

 renewing the water. 



A glass cylinder is filled with spring water into which, if it is 

 poor in free Carbon dioxide, we have led a small quantity of this 

 gas. We now bring into the fluid a twig of Elodea or Hippnris, 

 w r hich may be bound to a glass rod (see Fig. 14), and shall 

 observe that in the light bubbles of gas escape from the cut end 

 of the twig. The number of bubbles, which consist of air rich 

 in Oxygen, directly indicates the energy with which assimilation 

 is proceeding in the green twig. In direct sunlight, for ex- 

 ample, Elodea twigs frequently liberate such a rapid stream of 

 fine bubbles that we cannot count them, or can only do so with 

 difficulty. Other twigs of the same plant under the same con- 

 ditions assimilate less energetically. The bubbles of gas escaping 

 from the cut ends of Hippuris twigs are fairly large, and do not 

 appear in such excessive numbers. 



Without going into details regarding the relations between the 

 intensity of the light on the one hand and the energy of assimi- 

 lation on the other, it is easy to ascertain that the evolution of 

 Oxygen by green plant structures proceeds more rapidly in bright 

 diffuse light than in more feeble light. A shoot of Elodea bound 

 on a glass rod is exposed under water to the influence of bright 

 diffuse light. We count the number of bubbles liberated from 

 the cut end in a particular time, e.g., in one or in five minutes. 

 We next place a ground glass plate in front of the apparatus, 

 and it will be found that now fewer bubbles escape into the water 

 in the unit time than before. If we shade the twig well, the 

 assimilation, and hence also the production of Oxygen, falls off 

 entirely. 



Twigs of Elodea or Hippuris are exposed under spring water, 

 into which if necessary some Carbon dioxide has been led, to very 

 bright diffuse light. We count the number of bubbles escaping 

 in a definite time. Over the vessel containing the research 



