and Laboratory Methods. -1-9 



A control experiment supplied with carbon dioxid must be set up similar to 

 C, except that the water is not boiled nor passed through B, and the pumice 

 stone in the U-tube is wet with water instead of potassium hydroxid. Each day 

 carbon dioxid should be introduced into the control flask by opening the clamp 

 and forcing one's breath through the tube. At the end of three or four days 

 the plants are to be taken from the flasks, the chlorophyll extracted and tested 

 for starch. The plants will show that starch formation does not proceed in 

 the absence of carbon dioxid. 



10. The Relation of Light to the Evolution of Oxygen. The well known 

 formula, 6C0., ^oHjO^CijHj 1,0,5- GOo shows that six molecules of oxygen 

 are given off as a by product in the formation of a molecule of starch. It is 

 evident then that the rate of evolution of oxygen may be taken as an indication 

 of the amount of carbon assimilation. The evolution of the gas is easily ob- 

 servable in certain water plants. Fill a small, four-sided glass jar with water at 

 a temperature of •iO-22°C., during the course of the experiment the temperature 

 of the water must not vary outside of these limits. Select a vigorous stem of 

 FJodea canadensis 5 to 8 cm. in length and tie it to a glass rod. Pinch ofif the 

 proximal end of the stem with the fingers and insert rod and plant in the jar, 

 with the apex of the stem downwards. The jar is to be placed in direct sun- 

 light and the evolution of bubbles observed as they rise through the water. If 

 the bubbles do not come off in single file, or are too rapid to be counted, another 

 bit of stem should be pinched off. 



When the experiment is working well, place screens of thin, white tissue 

 paper or ground glass plates between the plant and the window, continue to re- 

 duce the amount of light until a further reduction would diminish the number 

 of bubbles. This may be designated as the optimum amount of light. In this 

 light, with the stop-watch in hand, determine the time required to produce a 

 given number of bubbles. Make at least five determinations and average them 

 for the final record. 



While the screens are still in position cover the plant with a double-walled 

 bell-jar containing a saturated solution of potassium dichromate and determine 

 the rate of evolution of bubbles. Replace the bell-jar with another containing 

 an ammoniacal copper sulphate solution and determine the evolution of bubbles. 



11. Dependence of the Evolution of Oxygen on Carbon Dioxid. Pour out the 

 water from the jar used in the last experiment, and fill the jar with water from 

 which the carbon dioxid has been removed as in flask B, Fig. 7. The tube 

 through which the water enters should reach to the bottom of the jar containing 

 the plant, and all agitation of the water should be avoided. The evolution of 

 bubbles in the sunlight should be observed ; then the observer may, by means 

 of a tube, force his breath through the water. Notice that the evolution of 

 oxygen is accelerated by the introduction of carbon dioxid. 



University of Michigan. HoWARD S. ReED. 



