SATURATING LIGHT INTENSITY 



987 



want to quote now is that the saturation rate of umbrophiHc plants usu- 

 ally is much lower than that of the heliophilic plants, even if related to 

 unit volume or unit area (not to speak of the rate per unit chlorophjdl con- 

 tent). This indicates that adaptation to weak light involves, in addition 

 to an increase in pigment concentration, a decrease in the amount of one or 

 several catalysts that exercise a rate-limiting influence in photosynthesis. 

 Coupled with steeper initial rise, this lower "ceiling" on the rate of photo- 

 synthesis in shade plants often leads to a very early light saturation. While 

 the light curves of sun-adapted plants may continue to rise at or even be- 

 yond 100 klux (cf. data of Singh and Kumar, Smith, Boysen- Jensen, and 

 Gabrielsen in Table 28.1), the light curves of shade plants may show 

 saturation at light intensities as low as 1 klux (cf. figs. 28. IG and 28.18). 



2- 



I ' 



o 

 If) 



Peltigera 



CSJ ^(J 



O 



o 



a. -I 



E 



0,^ 



Marchantia 



_L 



9 II 13 

 klux 



15 



17 19 21 23 



Fig. 28.16. Light curves of net gas exchange of an umbrophiHc moss (Mar- 

 chantia) and a hehophiUc hchen (Peltigera) (after Boysen-Jensen and IMiiller 1929). 

 The former is hght-saturated at 1 klux; the latter at or above 20 klux. 



The difference between the shapes of the light curves of heliophilic and 

 umbrophilic land plants was first observed by Weis (1903), who compared 

 the shade plant Poly podium with the sun plant Oenothera. This phenom- 

 enon was also investigated by Lubimenko (1905,1907,19081.2,1928,1929), 

 Boysen-Jensen (1918, 1929), Boysen-Jensen and Miiller (1929^) and Lunde- 

 gardh (1921, 1922), among others. Typical results are illustrated by 

 figures 28.16, 28.17 and 28.18. The first of these figures refers to an umbro- 

 philic moss (which is compared with a heliophilic lichen) ; the second com- 

 pares shade-adapted specimens of two aquatic plants with sun-adapted 

 individuals of the same species and the third contains a comparison of the 

 light curves of a shade-adapted leaf and a sun-adapted leaf on the same 

 plant (see also Table 28. IV). We see that the effect of phylogenetic adap- 

 tation (fig. 28.16) is similar to that of the individual adaptation of whole 



