THE AUTOMATIC RECORD 159 



value of photosynthesis at the particular hour. The plant 

 was exposed to light two minutes earlier for preliminary 

 accommodation to light. 



The plant might have been continuously exposed to 

 light and the records taken throughout the day ; but it 

 was exposed to light for only seven minutes at a time, in 

 order to guard against the possibility of depletion of C0 2 

 by uninterrupted photosynthesis for twelve hours. As the 

 quantity of solution contained in the apparatus was more 

 than 200 c.c, the depletion would have been slight even 

 after such continuous exposure to light ; it was nevertheless 

 thought desirable to adopt the safer course of giving an 

 exposure only long enough to make the observations at 

 definite periods. There was a simple contrivance (not 

 shown in the figure) for giving the necessary exposure. 

 An opaque cylindrical cover cut off the light from the plant- 

 vessel ; the pressure of a lever projecting outside the glass 

 cover enabled the observer to give the exposure for the 

 definite duration of seven minutes. 



The Electric Photometer was exposed to light by 

 manipulating a string which actuated the shutter ; the 

 intensity of light at the hour was found by observing the 

 maximum deflection of the galvanometer which was reached 

 in the course of five seconds. The temperature at the 

 hour was recorded by the Thermograph ; an independent 

 reading of the thermometer inside the case was also taken 

 at the same time. 



The Automatic Record 



The variation in the rate of photosynthesis is shown in 

 a very striking manner by the automatic records given in 

 fig. 44, for successive periods of five minutes. The sun rose 

 at 6.46 a.m., but the light was too feeble to be effective. 

 At 7.30 a.m. photosynthesis was initiated at a rate of 

 135-8 c.mm., the intensity of light indicated by the Electric 

 Photometer being 61, and the temperature 19 C. : there 



