986 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



proximately 60 klux in moderate zones; cf. chapter 25). However, the 

 saturating intensity varies widely from species to species and specimen 

 to specimen. One reason for this is difference in optical density. Satura- 

 tion begins when the most exposed chlorophyll molecules receive a certain 

 hght flux, and becomes complete when the most deeply shaded molecules 

 obtain this saturating intensity. The intensity of incident light at which 

 this complete saturation occurs obviously must depend on whether we use 

 a thick or a thin leaf, a dense or a dilute cell suspension. This "density 

 effect" already was described in chapter 25 and will be again discussed 

 later in this chapter (page 1007). 



Even with the density effect eliminated — either experimentally, by 

 using optically very thin objects, or by calculation {cf. fig. 28.22) — the 

 saturating light intensity still remains dependent, for a given species, on 

 internal factors such as age and adaptation (to strong or weak light), and 

 external variables, such as carbon dioxide supply and temperature. The 

 effects of carbon dioxide concentration are illustrated by figures 28.1 to 

 28.5, those of temperature by figures 28.6 to 28.8. Using the notion of 

 "ceilings" introduced in chapter 26 (page 869) we can say that everything 

 that lowers the ceiling imposed on the over-all reaction of photosynthesis 

 must shift the saturation toward lower light intensities. This may be a 

 decrease in [CO2], a decrease in available reductants (in purple bacteria), 

 or a decline in the amount of one of the catalysts. The temperature effect 

 is complex, because changes in temperature affect all ceilings simultane- 

 ously — those imposed by diffusion, as well as those caused by enzymatic 

 reactions of limited maximum yield. 



Among the internal factors affecting the saturating light intensity, the 

 most important is adaptation to strong or weak light. Shade-adapted 

 plants often are darker green, i. e., contain more chlorophyll (per unit area 

 or unit volume) than the corresponding light-adapted species or individuals. 

 This difference in optical density would in itself be sufficient to cause 

 changes in the shape of the light curves: Darker, shade-adapted plants 

 are more efficient light absorbers, and their light curves should there- 

 fore have a steeper initial slope. If the higher optical density is due to in- 

 creased concentration of the pigment (with the concentration of all other 

 constituents of the catalytic apparatus remaining the same), the saturation 

 rate, related to unit volume of cells (or to unit area of leaves, assuming the 

 leaf thickness is constant), should be the same for heliophilicandumbrophilic 

 varieties; while the saturation rate related to U7iit amount of chlorophyll 

 should be lower in the darker specimens. In practice, conditions are more 

 complicated, because shade leaves often are thicker, and shade cells do 

 grow larger than their heliophilic counterparts. These relationships will 

 be discussed in more detail in chapter 32. The experimental result we 



