144 THE BIOLOGY OF FLOWERING PLANTS 



times that of direct sunlight, or when the carbon dioxide 

 concentration is over three volumes per cent. For land 

 plants, constant rates of assimilation have been obtained 

 with as much as lo per cent, carbon dioxide. It is known 

 that very high light intensities have an injurious effect on 

 the protoplasm and on the chlorophyll, while carbon 

 dioxide acts as a narcotic. But in natural conditions, neither 

 light nor carbon dioxide content can ever retard assimilation. 



Limiting Factors. — The second fundamental advance 

 made by Blackman is his analysis of the way in which these 

 three factors interact. He states as an axiom : "When a pro- 

 cess is conditioned as to its rapidity by a number of separate 

 factors, the rate of the process is Hmited by the pace of the 

 ' slowest ' factor." This is often referred to as the theory 

 of limiting factors. The effect of this in practice may be 

 illustrated by considering the case of a plant assimilating 

 in ordinary air under increasing illumination. As the light 

 gets stronger the rate of assimilation rises until a certain 

 point is reached, at which no further rise takes place however 

 much stronger we make the light. Now this does not mean 

 that we have reached a point at which increase of light, as 

 such, has no further effect ; it means that at this point the 

 carbon dioxide supply is being completely used up. Carbon 

 dioxide supply is now a limiting factor and increase of 

 light is without effect, because the materials of assimilation 

 are lacking. If we artificially supplemented the carbon 

 dioxide supply, we should find that increase of light would 

 again give higher values. 



In this account we have neglected temperature, but in 

 practice it must of course be included. Suppose our object 

 of study is a plant in the open air, and we are following its 

 rate of assimilation in the early morning. We might well 

 find that increase of light and increase of carbon dioxide 

 both failed to give higher rates of assimilation, in which 

 case the temperature would be acting as the limiting factor. 

 The rate at which a plant assimilates (assuming that it is 

 well supplied with water and has its stomata open) is always 

 limited either by temperature, by carbon dioxide supply, 



