222 Plant Physiology 



account of darkness, but is to be continued later, close pinch- 

 cock (G) and refill with water the (DE) end of the series, before 

 continuing the experiment. 



Use of the bubble-counting method to show rate of photosynthesis. 

 A general idea of the relative rate of photosynthesis under 

 different conditions may be obtained by counting the number 

 of bubbles of oxygen evolved in the same space of time. 



a. Method. With a rubber band attach a freshly cut sprig 

 of Elodea or other water plant to a glass rod and submerse in a 

 large test-tube of water at laboratory temperature, or somewhat 

 above 20 C. Water from the tap generally contains sufficient 

 CO 2 , but in long-continued experiments it may be necessary to 

 lead in some CO 2 from a generator. Place the tube as above 

 prepared in a wire rack in direct sunlight, and after a few minutes 

 ascertain if the bubbles escape uniformly ; also the average num- 

 ber given off in a unit of time, say one minute. If the bubbles 

 do not come off with sufficient uniformity, try another shoot, or 

 seal the cut end of the stem with wax, and then pierce a hole 

 through the latter with a small needle. 



6. Light intensity. When a careful count of the bubbles 

 has been made in direct sunlight, remove the tube to light suc- 

 cessively weaker ; note any change in the rate, and determine 

 where the evolution of gas ceases. If possible, contrast the light 

 intensity at this point with that of the open window, as a stand- 

 ard, using an ordinary photographic actinometer. 



c. Temperature. After counting in direct light the number 

 of bubbles given off when employing water at laboratory temper- 

 ature, transfer the sprig promptly to water brought to a tem- 

 perature of from 2 to 3 C., but otherwise similar to the preceding. 

 After allowing a few minutes for adjustment, make observation 

 upon the rate of O 2 evolution promptly, before the sunlight has 

 had an opportunity to raise the temperature appreciably ; then 

 warm the tube gradually to 20 or 25 C. and note the result. 



d. If time permits, determine by the bubble-counting method 

 the rate of photosynthesis under blue and under orange-yellow 

 screens, employing apparatus described in Chapter XVII. 



A simple test for starch. Make a small quantity of a very 



