348 



FACTORS AFFECTING PHOTOSYNTHESIS 



will be discussed under these three headings. Before the various effects of 

 light upon photosynthesis are considered, however, it will be desirable to 

 analyze the physical relations between leaves and incident light. 



I. The Optical Properties of Leaves. — Of the visible light which falls 

 on leaves, as with radiant energy generally, a part is reflected, a part is 

 transmitted through the leaf, and a part is absorbed by the leaf. The pro- 

 portion of the visible light incident upon a leaf which is absorbed varies 

 considerably according to the kind of leaf and the intensity of light but is 

 frequently in the neighborhood of 8o per cent (Seybold, 1932) and probably 

 often higher. 



For any given leaf the proportion of the incident light reflected varies 

 considerably according to wave length (Shull, 1929). In all normally green 



430 



500 540 580 620 



WAVE LENGTHS IN MILLIMICRONS 



Fig. 86. Per cent reflection of different wave lengths of light from leaves of lilac 

 {Syringa 'vulgaris). {A) from lower surface, {B) from upper surface. Data of 

 Shull (1929). 



leaves, however, maximum reflection apparently occurs in the range of wave 

 lengths corresponding to the green portion of the spectrum (Fig. 86). 



Similarly the proportion of incident light of each wave length which is 

 transmitted varies considerably depending upon the kind of leaf. For normally 

 green leaves transmission is relatively high in the green, relatively low in the 

 blue violet and short red, and greatest in the long red (Fig. 87). The green 

 color of leaves is due to the proportionately greater reflection and transmis- 

 sion of light in the green region of the spectrum. 



2. Effects of Variations in the Intensity of Light upon the Rate of Photo- 

 synthesis. — The ei¥ect of various light intensities upon the rate of photos}'n- 

 thesis of wheat plants exposed to different atmospheric concentrations of carbon 

 dioxide is depicted graphically in Fig. 88. In general with increase in light 

 intensity there is an increase in the rate of photosynthesis until some other 

 factor, in this example, the carbon dioxide concentration, becomes limiting. 

 At relatively low light intensities, as long as carbon dioxide is not the limit- 



