PHYSIOLOGICAL STUDIES 49 



been considered solely in terms of the concentration of 

 carbon dioxide (together with undissociated carbonic acid). 

 In Chlorella the rate of photosynthesis both at high and low 

 concentrations of carbon dioxide appears to be independent 

 of pH. Thus there is no evidence that bicarbonate ions 

 (which are in greater concentration the more alkaline the 

 solution) are decisive in determining the rate of photosyn- 

 thesis. It seems unlikely that this is true for all plants as 

 there are some indications that in ScenedesmuSy for example, 

 a more complex situation may exist. 



Light intensity. Other measurements have been concerned 

 with the relationship between rate of photosynthesis and the 

 intensity of illumination when the concentration of carbon 

 dioxide is maintained high. Owing to the physical structure 

 of the leaf, the intensity of illumination at a point within the 

 plant will be a function of its depth from the surface; the 

 quality (or colour) of the light will also vary owing to the 

 selective absorption by the cells between it and the light 

 source. Moreover within a single cell the absorption may 

 change as a result of the movement of the chloroplasts, as 

 for example in Funaria, in which the chloroplasts assemble 

 on the side of the cell nearest a source of moderate intensity, 

 but in high or more weak intensities distribute themselves 

 in the less brightly illuminated portions (see Fig. 4.4). Pre- 

 cautions must also be taken when using leaves to ensure that 

 the temperature of the leaf does not become higher with 

 higher intensities of illumination. Some of these difficulties 

 are absent or present to a smaller degree when suspensions 

 of algae are used, and we shall consider only measurements 

 with these plants. With Chlorella the relationship between 

 rate of photosynthesis and light intensity is again approxi- 

 mately of the Michaelis type, as shown in Fig. 4.5. Just as in 

 the case of carbon dioxide concentration we may then sug- 

 gest that the simplest possible view is that light energy is 

 involved in a process which results in a chemical change in 

 an acceptor molecule. There is no evidence that chlorophyll 

 undergoes any change during photosynthesis, and hence it is 

 necessary to postulate a substance in addition to chlorophyll 

 which is primarily concerned with the light reaction. 



