96 



PHOTOSYNTHESIS 



act as the limiting one and thus determine the rate of photosynthetic 

 activity. 



There has been some debate and misunderstanding regarding this 

 theory. Below is given Blackman's original illustration. "Suppose a leaf 

 in a glass chamber to have enough light falling upon it to give energy 

 equal to decomposing 5 cc. of carbon dioxide per hour. Then, as one 

 gradually increases the carbon dioxide in the air current through the cham- 

 l)er from the amount (or pressure) that causes 1 cc. to diffuse into the 

 leaf through its stomata up to five times that pressure, so steadily the 

 assimilation will increase from 1 cc. to fivefold. After that, further in- 

 crease of carbon dioxide will produce no augmentation of the assimilation. 



Fig. 7. — Blackman's conception of the effect of limiting factors on the rate 



of photosynthesis. 



but will give continually an effect of 5 cc. of carbon dioxide — the light 

 being now the limiting factor. The curve obtained will be of the form 

 ABC. (Figure 7.) 



"Ultimately, if the supply of carbon dioxide in the air current be 

 increased up to 30, 50, 70 per cent, the carbon dioxide will have a 

 general depressing effect on the whole vitality, and before suspension 

 of all function a diminution of assimilation undoubtedly occurs ; this 

 is, however, quite a separate process. Now, secondly, suppose light 

 falling on the leaf to be sufficient for the decomposition of 10 cc. of 

 carbon dioxide per hour, then twice the external pressure of carbon 

 dioxide will be required to reach the limit and the angle of the curve, 

 which will now be A B D E. With still stronger light we should get 

 A B D F G. Those who would be prepared to admit that a curve like 

 ABC shows an optimum, only with a very long drawn-out top, would 

 have to further admit that for each intensity of light falling on a leaf there 

 is a different optimum amount of carbon dioxide. This is not to be 

 entertained. 



