l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 87 



variations of intensity through the leaf. Rapid recirculation of the 

 air insures a small variation in concentration of carbon dioxide over 

 the plants. 



In view of these precautions, is it possible to explain the whole 01 

 the transition range as being due to variations in the environmental 

 conditions? The actual variation of intensity over the surface of the 

 leaves, as determined by thermocouple and photronic cell measure- 

 ments, may be as much as 10 or 12 per cent from the mean values 

 given. The chloroplasts are distributed more or less uniformly 

 throughout the entire body of the wheat leaf ; thus, there must be a 

 considerable variation in intensity when we consider chloroplasts at 

 different distances below the surface of the leaf. In order to get 

 some idea of the absorption of the light in a leaf, the transmission 

 of a wheat leaf was measured with a photronic cell, a 500-watt lamp 

 being used as the source of light. The transmission of the leaf for 

 the whole range to which the cell is sensitive was 18 per cent. When 

 the radiation from the lamp was filtered through a heat-absorbing 

 glass the transmission was reduced to 14 per cent. This is due, of 

 course, to the regions of selective absorption. Even with the leaf 

 illuminated from both sides it must be concluded that there is a 

 large variation in the intensity of radiation at the surfaces of the 

 various chloroplasts throughout the leaf. If one assumes that the 

 whole transition range may be explained by a variation in light in- 

 tensity then the variation in the intensity may be determined from 

 Figure 8. For a concentration of carbon dioxide of about 900 we 

 see that light is the limiting factor for a very small range. At an 

 intensity of 10, carbon dioxide has already become the limiting factor 

 for some of the chloroplasts. It is necessary, however, to increase 

 the intensity to 200 or more before carbon dioxide is the limiting 

 factor for all the chloroplasts. This would indicate that some of the 

 chloroplasts were receiving only 5 per cent or less of the effective 

 radiation intensity of others. It does not seem likely that the variation 

 of intensity is so great. 



Comparing these families of curves obtained from young wheat 

 plants with those of Van den Honert obtained from algae one observes 

 a close agreement as to general form. Both show well-defined linear 

 variations over restricted ranges ; both show transition from one 

 linear range to the other in which the two variables affect the photo- 

 synthesis. Higher plants show a wider range of transition, as is to 

 be expected from the fact that both light intensity and carbon dioxide 

 concentration vary over a considerable range for different chloro- 

 plasts. Whereas light intensity has been measured with considerable 





