J262 THE PIGMENT FACTOR CHAP. 32 



No "trivial" interpretation can be suggested for the behavior of 

 shade-adapted plants in strong light. As stated on page 986, and illustrated 

 by figures 28.16-28.18, these plants do not approach the same saturation 

 values of photosynthesis as the heliophilic plants. (In other words, they 

 do not conform to the behavior predicted in figure 32. IB for systems that 

 differ only with respect to light absorption.) To the contrary, the photo- 

 synthesis of umbrophilic plants or leaves usually becomes light-saturated 

 on a much lower level than the photosynthesis of plants or leaves adapted 

 to strong light. Many umbrophiles suffer light inhibition (or permanent 

 injury) whenever the light intensity is increased even a little beyond the 



saturating value. ^^ 



If the light curves of all shade plants were of this "optimum type 

 (lowest curve, fig. 28.19), one could consider their low saturation as ''ap- 

 parent," by assuming that light inhibition occurs in these plants before 

 true light saturation has been reached. However, the light curves of many 

 shade plants, e. g., those shown in figures 28.16 and 28.17, show an extended 

 saturation plateau. We must assume that these umbrophiles, while con- 

 taining more chlorophyll than the corresponding heliophiles, contain less of a 

 catalyst that limits the rate in intense light (such as Franck and Herzfeld's 

 "stabihzing catalyst," Eb)- 



This antiparallelism between [Chi] and P'"'^^', often found m the com- 

 parative study of sun plants and shade plants, is, however, by no means a 

 general rule. Already in Table 28.V, we saw that, according to Willstatter 

 and Stoll (1918), many plants with widely different chlorophyll contents 

 have almost identical "assimilation numbers," va] in other words, their 

 rate of photosynthesis in intense light (about 45,000 lux) and in presence of 

 abundant carbon dioxide (5%) is directly proportional to their content of 

 chlorophyll. The ratio va = P"^"/[Chl] (with P^^^' measured in grams 

 carbon dioxide consumed per hour per gram chlorophyll present) is, for 

 those plants, called "normal" by Willstatter and Stoll, of the order of 6 to 

 8. The Va values have been found to have this order of magnitude not only 

 in many higher land plants, but also in a number of algae. For example, 

 according to van den Honert (1930), the unicellular alga Hormdmw has an 

 assimilation number of 6.8; Emerson (1935) found values between 4 and 8 

 for Chlorella. The pa values given by Noddack and Kopp (1940) for the 

 same species were somewhat but not much lower {va = 4.5 for a suspension 

 grown in strong light and 2.6 for a shade-adapted suspension). Gessner 

 (1943) studied the phytoplankton populations of fourteen Bavarian lakes, 

 and found in twelve of these populations, consisting of varying proportions 

 of Chlorophyceae, Cyanophyceae and diatoms, at different times of the 

 year, va values between 4.2 and 6.2. The plankton from one lake gave 2.6 

 and that from another, 9.0 ; but these were single determinations of doubt- 

 ful precision. 



