LIMITING FACTORS 147 



increase of temperature may cause a small increase of 

 assimilation by increasing the rate of diffusion of the carbon 

 dioxide." This is, of course, a very different position from 

 that taken up by Lundegardh, who holds that, for each 

 higher light intensity the graph connecting assimilation 

 rate with carbon dioxide supply rises at a steeper angle. 

 Lundegardh's results may, of course, be due to some 

 secondary effect, such as the degree of stomatal opening. 

 It must, however, be emphasised that Blackman has 

 established a method of experiment and of analysis to 

 which all work on photosynthesis must conform if it is to 

 be exact. 



Nutrient Salts. — Briggs (1923) has shown that, if plants 

 are grown in culture solutions lacking either potassium, 

 magnesium, phosphorus, or iron, the rate of assimilation 

 does not reach the value shown by plants grown in a complete 

 culture solution, which again does not reach that shown by 

 plants grown in soil. The lower rate is exhibited when 

 calculated either for unit leaf area or for unit leaf weight, 

 and is not therefore an effect of lessened development of 

 leaf surface, which was indeed not markedly different in 

 the different plants. Briggs supposes that the diminution 

 is due to a failure to develop the full reactive surface in the 

 chloroplast. Lack of nutrient salts, therefore, directly 

 affects plant growth through its action on photosynthesis. 

 An indirect effect will also be produced, since the diminished 

 assimilation ultimately means a diminished assimilating leaf 

 surface. 



§ 17. Assimilation in Natural Environment 



We may now consider the actual effects of changes in 

 these conditions on the rate of assimilation in nature. The 

 action of light and of heat are closely linked because they 

 tend to vary together. Their relations have been studied 

 by Blackman and Matthaei (1905), and by Thoday (1910). 

 Blackman 's experiments were carried out on leaves of the 

 cherry laurel, and of the artichoke, a plant with leaves very 



