INTERACTION OF FACTORS 



II 



It will be seen that the intensity of carbon assimilation 

 increases both in the vertical and the horizontal directions, 

 i.e. with increase in carbon dioxide and with increase in illu- 

 mination. Harder concludes that a single factor appears to 

 be limiting only when it is very much weaker than the other 

 factors. Its action is not directly proportional at all con- 

 centrations or intensities of the other factors, thus the higher 

 the light intensity, the greater is the augmenting effect of an 

 increase in the concentration of carbon dioxide. That is, 

 both of these factors become interlocked and play a part, 

 so that the rate of carbon assimilation is dependent on the 

 concentration of carbon dioxide as well as the intensity of 

 illumination. 



Lundegardh * found that shade plants such as Oxalis 

 acetosella growing in an atmosphere containing the normal 

 amount of carbon dioxide and in varying intensities of illu- 

 mination gave a curve of the abrupt type, for in light above 

 one-tenth of full sunlight no increase in photosynthesis was 

 observed unless the amount of carbon dioxide were increased. 

 Sun plants such as Nasturtium palustre, on the other hand, 

 behaved differently : with low light intensities photosyn- 

 thesis was proportional to the illumination, but with increasing 

 light, photosynthesis was slower than the increase in carbon 

 dioxide concentration ; that is, a proportional relationship 

 no longer obtained. And in an atmosphere relatively rich 

 in carbon dioxide, carbon assimilation was observed even in 

 light of an intensity one-fortieth that of direct sunlight. In 

 such instances, the curves are of the logarithmic type and 



* Lundegardh : " Svensk. Bot. Tidsk.," 192T, 15. 4°- 



