730 



LIGHT ABSORPTION BY PIGMENTS IN VIVO 



CHAP. 22 



carotenoids. Above 700 m/x, the absorption by purple bacteria is due en- 

 tirely to bacteriochlorophyll, as shown in figure 22.26 by comparison of the 

 cell spectrum with the spectrum of the extracted green pigment. 



Appendix. Natural Light Fields* 



The importance of the various pigments in the light economy of plants 

 cannot be fully understood on the basis of curves discussed in the preceding 

 section, because it also depends on the spectral composition of the light 

 available to the species or individual plant under natural conditions. 



>- 

 o 

 on 



UJ 



UJ 



> 



UJ 



q: 



Fig. 22.50. Energy distribution 

 curves for solar radiation at differ- 

 ent heights of the sun over the hori- 

 zon (after Seybold 1936). For 38°, 

 21° and 14° curves, black square 

 = 2.50 X 10-3 cal./(cm.2 min.); 

 for 10° to 2 "curves, black square 

 = 0.25 X 10-3 cal./(cm.2 min.). 



If) iDO O O OO O If) O O 



U) tOif) (J) OJ iDh- O in OD — 



ro ^"^ t m if)io ID IS) IS) ^ 



WAVE LENGTH, m;i 



Plants live in "light fields" whose normal character depends on the clima- 

 tic zone and habitat, and which are also subject to variations with the 

 season of the year, the time of day and the meteorological conditions. 

 Three characteristics of natural light fields are important to plants : their 

 total intensity, their spectral composition and their periodicity. There are 

 strong indications that plants adapt their life processes in general, and their 

 photosynthetic apparatus in particular, to all these three factors. The 

 intensity adaptation reveals itself in the different characters of "umbro- 

 philic" and ''umbrophobic" (or "heliophilic") plants (shade and sun plants); 

 chromatic adaptation is most strikingly shown by the occurrence of red algae 



Bibliography, page 738. 



