982 



THE LIGHT FACTOR. 



INTENSITY 



CHAP. 28 



One could also call "compensation point" the carbon dioxide concentration at which 

 the gas exchange becomes zero at a given light intensity {cf. chapter 27) ; or the tem- 

 peratui-e at which the gas exchange becomes zero at a given combination of the param- 

 eters I and [CO2] (cf. chapter 31); but the word is seldom used in either of these two 

 ways. Sometime, the designation "upper compensation point" is applied to the second 

 crossing of the curves of photosynthesis and respiration, which may occur either at very 

 high light intensities or at "superoptmial" temperatures (cf., for example, fig. 31.1). 



When the carbon dioxide supply is not too low, compensation occurs 

 within the linear range of the light curve, where the slope of the latter is 

 determined by the maximum quantum yield of photosynthesis and the rate 

 of light absorption {i. e., the optical density of the specimen). Probably 

 {cf. chapter 29) the maximum quantum yield is approximately the same for 

 all species (at least, when all cells are fully active — which is not always the 

 case, e. g., in "aged" cultures). Differences in the compensation points 

 found under these conditions must therefore depend mainly or exclusively 

 on two factors: rate of respiration and optical density of the specimen. 



Respiration is proportional to the concentration of cells in a suspension ; 



Table 28.III 

 Compensation Point of Leaves and Thalli 



Authority 



Plant 



7 , lux Temp. 



HIGHER LAND PLANTS 



Boysen-Jensen, Mliller (1929) Fraxinus excelsior (shade leaves) 200 20° C. 



(sun leaves) 700 20° C. 



Fagus silvatica {shside leases) 150 20° C. 



(sun leaves) 500 20° C. 



MOSSES 



Boysen-Jensen, Miiller (1929) Marchavtia pohjmorpha 

 Stalfelt (1939^) 6 species (winter), average 



6 species (summer), average 



100 

 390" 



20° C. 



About 11° C. 



LICHENS (symbiotic GROWTHS OF ALGAE AND FUNGi) 



Boysen-Jensen, Miiller (1929) Peltiqera canina 4200 



Stalfelt ( 19391) 12 species (winter), average 1020 



12 species (summer), average 1160 



'' 20° C. 

 13° C. 



AQUATIC HIGHER PLANTS AND MOSSES 



Plaetzer (1917) 



