NO. 24 TIME COURSE OF PHOTOSYNTHESIS McALISTER 5 



shows that after about four " trips " around the system, i. e., in about 

 20 seconds, complete mixing is accompHshed. In following the re- 

 sponse of a plant, galvanometer readings are taken every 30 seconds, 

 including a " zero " reading. These readings are timed with a shutter 

 operated by a synchronous motor. The " zero " readings are neces- 

 sary to eliminate slow " zero drifts " that are inevitable in a galva- 

 nometer system of this sensitivity — i millimeter scale corresponding 

 to TO''' amperes. 



RESULTS 



With this speed of response and sensitivity in mind, let us examine 

 the reactions of three young wheat plants to light and darkness. 



Fig. 4.— Response of plant after a lo-hour period of darkness. 



Figure 4 shows a typical set of data. Ordinates, which are roughly 

 inversely proportional to the carbon dioxide concentration in the sys- 

 tem, are galvanometer deflection in millimeters. Abscissae are time 

 in minutes. At the beginning of the experiment (plant in darkness), 

 air from a supply tank is blown rapidly through the system and read- 

 ings are taken every 30 seconds for 5 minutes. Then the system is 

 closed (time = 5), and we see respiration building up the carbon 

 dioxide concentration. This is followed for 10 minutes to establish 

 the rate, i. e., slope of the line. Next the plants are illuminated with 

 white light of 500 foot-candles from a Mazda lamp (time=i5). 

 They had been in darkness for the previous 10 hours. During the 



