14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



diffuse to the chlorophyll. Taking- a case where light is the Hmiting 

 factor (200 foot-candles), and assuming the concentration at the 

 chlorophyll to be one-third that in the surrounding air, we find that 

 0.9 second is the time required.^ This is small compared to the length 

 of the induction period (about i^ minutes) at this intensity (see 

 fig. 9). It appears then that this induction in a higher plant is a 

 process not structural or physiological, but fundamentally chemical. 



* In figure 9 the numbers written adjacent to the Hues showing the respiration 

 and assimilation (true) are cubic millimeters of COz per 10 minutes at 20° C. 

 For example, the value 750 on the top curve means that the uptake of CO2 

 (respiration added) is 75 cubic millimeters per minute. The leaf area (both 

 sides) was 360 cm^ The leaf thickness was 0.08 to 0.09 millimeters. The dif- 

 fusion equation is 



r. KA 



where 



^-(a-CO -t (I) 



F=volume of CO2, (cm') 

 A =^ cross-sectional area of path, (cm^) 

 L= length of path, (cm) 

 C2 = concentration (volume) at outer surface 

 Ci = concentration (volume) at inner surface 

 if = diffusion constant (CO2 in water) cmVsec. 

 = 1.8 X 10-' cmVsec. (from I. C. T.) 



/ =z time in seconds 



assuming 



Ci = (CO2 limiting factor) 

 C2 = 0.0003 (normal air) 

 and solving (I) for L we have 



L=^(C2-C0i 

 _ 1.8 X iQ-' X 360 X (0-0003) X 60 



.075 

 = 15 X 10 * cm or 15 X 10'' mm 

 This is about i the leaf thickness. 



The time (calculation is as follows : From figure 9 for an illumination of 

 214 foot-candles (light limiting factor) there is an assimilation of 46 cubic 

 millimeters per minute, which is about 0.0008 cc per second. Equation (I) 

 solved for the time is 



LV 





KA(C.-G) 



IS X 10-" X .0008 



1.8 X 10"^ X 360 X (.0003 — .0001 ) 

 = 0.9 seconds 

 The dry weight of the leaves of the three plants used in this experiment 

 was 260 milligrams. Assuming i percent of the dry weight of the leaves is 

 chlorophyll we have, at 20° C, an assimilation number of 



■075 X -002 X 60 (mg of CO2 per hr.) _ 

 2.6 (mg of chlorophyll) 



At 30° C. this would be about 7.0. 



