ASSIMILATION OF CARBON • 5 



A less exact method consists in counting the number of gas bubbles 

 (comparatively pure oxygen 1 ) given off, in light, from the cut end of a piece of 

 the water plant Elodea, submerged in water saturated with carbon dioxide, 

 as shown in Fig. 2. If a number of such green water plants are placed under 

 water in sunlight and are covered by an inverted funnel, over the neck of 

 which is inverted a test-tube of water (Fig. 3), the test-tube soon becomes 

 filled with a gas that is nearly pure oxygen. 



Shiitzenberger's reagent (a solution of indigo carmine or nigrosine, de- 

 colorized by sodium sulphite) can also be used to demonstrate that oxygen is 



/--\ 



Fig. 1. — Leaf in 

 position in a measuring 

 tube, for demonstration 

 of absorption of carbon 

 dioxide and elimination 

 of oxygen during photo- 

 synthesis. 



Fig. 2. — Elimination 

 of oxygen bubbles by 

 Elodea in sunlight. 



Fig. 3. — Collection of oxygen from 

 water plants in light. 



liberated by water plants; this solution is yellow when prepared, but turns blue 

 in the presence of oxygen. If a shoot of Elodea, or other aquatic, is placed 

 in a dilute solution of this reagent and exposed to sunlight, the solution surround- 

 ing the leaves becomes blue in a few minutes. 2 [It is generally assumed that all 

 of the carbon dioxide decomposed by ordinary green leaves is derived directly 

 from the air by diffusion, but there are of course two other sources of this funda- 



1 This method was perfected by Kohl. See: Kohl, F. G., Die assimilatorische Energie der blauen und 

 violetten Strahlen des Spektrums. Ber. Deutsch. Bot. Ges. 15: 111-124. 1897. 



2 Kny, L., Die Abhangigkeit der Chlorophyllfunction von den Chromatophoren und vom Cytoplasma. 

 Ber. Deutsch. Bot. Ges. 15:388-403. 1897. [See also: Kolkwitz, R., Pflanzenphysiologie. Jena, 1914- P- 3-1 



