SPECTRA OF PHOTOSYNTHETIC PIGMENTS 351 



great inaccuracies in the measurement of absorption. An even greater 

 difficulty is the overlapping of the absorption spectra of carotenoids and 

 chlorophyll in the blue-green parts of the spectrum. For the purpose of 

 evaluating the absorption due to the different pigments in leaves, these 

 regions have thus far been nearly completely unusable. In fact, in this 

 region it is often impossible even to recognize the appearance of distinct 

 absorption peaks of pigments that are known to be present. 



Let us first consider the absorption, reflection, and transmission spectra 

 of a white leaf containing no chlorophyll. The striking thing about a 

 "white" leaf, illustrated in Fig. 6-2a, is the high absorption, coming to 

 nearly 50 per cent at the blue end of the spectrum and dropping to only 

 about 20 per cent in the red. This may be taken as a very rough esti- 

 mate of a base line to which in normal green leaves are added the absorp- 

 tion spectra of the chloroplast pigments. The reflection from this white 

 leaf is about 50 per cent through most of the spectrum, a value that is 

 reached only in the infrared by leaves that contain pigments. In nor- 

 mally pigmented leaves the reflection is greatly reduced and the absorp- 

 tion greatly increased in comparison with this white leaf. Since the total 

 of the absorption, reflection, and transmission is equal to 1, the relation 

 between these when the absorbing power is increased can easily be visu- 

 alized. This particular leaf of Fig. 6-2a is somewhat unusual in being 

 completely free of chlorophyll. Most "white" leaves actually contain 

 a small amount of chlorophyll which can usually be seen clearly by obser- 

 vation of either absorption or fluorescence with a spectroscope, even 

 though the green color is not obvious to the eye. In general, absorption 

 throughout the spectrum, as illustrated by this white leaf, is largely 

 responsible for the much higher absorption of leaves in the green part 

 of the spectrum than would be predicted from the known absorption 

 spectra of chlorophyll and the carotenoid pigments. The effect of this 

 broad-band absorption throughout the spectrum upon photosynthesis 

 measurements is discussed by Strain (1950). Measurements of a thin, 

 light-colored lettuce leaf are presented in Fig. 6-2c. In the infrared the 

 reflection and absorption of this leaf containing chlorophyll are much 

 like those of the previously discussed white leaf, since the chloroplast 

 pigments do not absorb in that region. This high reflecting power of 

 leaves in the near infrared as compared with the high absorption in the 

 visible part of the spectrum is the basis of the camouflage problems that 

 depend upon matching the absorption and reflection spectra of paint 

 with those of leaves. Ordinary green paint, although it may have 

 absorption in the visible part of the spectrum rather similar to that of 



* Chlorophyll a Chlorophyll b Carotene Xanthophyll 



Top curve 2.04 0.55 0.12 0.59 



Middle curve 2.7 0.64 0.21 0.80 



Lower curve 0.8 0.05 0.08 0.48 



