TRANSMITTANCE AND REFLECTANCE OF LEAVES 683 



that the whole phenomenon is unrelated to photosynthesis, since in natural 

 light fields, red light does not occur without the presence of some blue- 

 violet hght as well. 



Detlefson (1888) and Purevich (1914) found that leaves become less transparent 

 in the presence of carbon dioxide, but Ursprung (1918) was unable to confirm this result. 

 This effect, if at all real, could be attributed to increased formation of starch grains. 



Scattering of light in leaves can be decreased by injection of water into 

 the air channels (e. g., by evacuation under water; cf. figs. 22.8 and 22.9). 

 According to Seybold (1933-), water-filled leaves of land plants (as well as 

 those of aquatic plants of the type of Elodea) transmit about twice as much 

 light as air-filled leaves. For example, a submerged leaf of Potomageton 

 alpinus was found to transmit as much as 22% of white light (Seybold 

 1932); similar figures were obtained by Seybold (1934) for the transmis- 

 sion by algae {Chloj-ophyceae, Phaeophyceae and Rhodophyceae). In- 

 creased transparency of water-filled leaves or thalli is due to reduced dif- 

 fuse reflection (5% instead of >10%), rather than to weaker absorption. 

 As a rough approximation, it can be assumed that average land leaves 

 transmit 10% of (infrared-free) white light (400-700 m/x), reflect 10%o and 

 absorb 80% ; while average aquatic plants or algal thalh reflect 5%, trans- 

 mit 15% and absorb 80%. 



Loomis (1947) found, for 28 species of leaves, transmissions between 2 

 and 9%, and reflections of the same order of magnitude. Yellow tobacco 

 leaves transmitted -^ 30% of visible light, and light yellow leaves, 36% of 

 visible and 53% of total sunlight. Normal leaves of Nicotiana and Quercus 

 transmitted 5-7% of visible, and 25-30% of total sunlight. 



According to Seybold, a single chloroplast transmits from 30 to 60% 

 of visible light (depending on spectral distribution), absorbs 30 to 60% and 

 reflects about 10%. As mentioned in chapter 19 (Vol. I) an average leaf 

 contains the equivalent of from five to ten complete layers of chloroplasts; 

 these can easily account for the above-estimated absorption of 80% of the 

 incident white light. For a detailed discussion of the light absorption in 

 successive layer of chlorophyll molecules or plastides, see Seybold and 

 Weissweiler (1943). 



In regularly patterned systems of colored and colorless materials, the absorption 

 sometimes depends upon the angle of incidence. WTien this is the case the absorption of 

 diffuse light may be different from that of collimated light. Seybold (1933'') made a 

 search for such an effect in leaves, but without suc^esSi The reason probably is that 

 leaves are such strong scatterers that collimated light is converted into practically com- 

 pletely diffuse light, long before it emerges from the leaf. 



