400 



W. L. BUTLER 



quired for the gas to flow from the leaf to the analyzing chamber. 

 The inertia and time delay of this system, although less than mano- 

 metric measurements, preclude measurements as rapid as those of 

 Brackett and Gaffron. 



In order to determine the reading which corresponded to the com- 

 pensation level on these curves (i.e., the point at which the gas ex- 

 change of respiration is compensated by that of photosynthesis), the 

 gas flow was shunted aroimd the leaf. With such a bypass there is no 

 change in gas composition. 



^^C02 ^°2-C02 



Fig. 1. Rate of gas exchange vs. time for a hydrangea leaf at room temperature in 

 an atmosphere of 4% CO2 and 20% O2 in He. 



Since the apparatus is somewhat more sensitive to CO2 changes than to O2 

 the displacement of the curve due to the CO2 assimilation is greater than that due 

 to the O2 evolution during the steady state when the assimilatory quotient is imity. 



The steady-state rate of O2 evolution and CO2 uptake is 18 lA/min. This changes 

 the composition of the gas from 4% CO2 and 20% O2 to 3.964% CO2 and 20.036% 

 O2. 



In most of the work the under side of hydrangea leaves was irradi- 

 ated. It was determined that the stomata are all on the under side of 

 the leaf and are always open. 



Figure 1 shows a typical set of curves obtained with 4% CO2 

 and 20% O2 in He. The initial shoulder in the O2 curve at or slightly 

 above the compensation level is a general feature of these curves, pro- 

 vided the previous dark period has been at least 5 or 10 minutes. The 

 shoulder is independent of light intensity provided the steady-state 

 rate of photosynthesis is somewhat higher than compensation. At 

 low CO2 pressures, the shoulder remains at compensation, although it 

 becomes less prominent because the induction period is of shorter 

 duration. 



