LIGHT AND VEGETATION 55 



longer than 2 /x the leaf becomes more and more "black" 

 (Fig. I, 12). 



Light-coloured leaves, deficient in chlorophyll, behave 

 very differently from green leaves. The explanation given in 

 the preceding paragraph would lead us to expect that the 

 absence of chlorophyll will have the effect of extending, 

 from the near infra-red to the green blue in the visible, the 

 range of the high reflection factor, about 50 per cent, which is 

 manifested when chlorophyll does not absorb. This is actually 

 what takes place. 



The transmission factor also remains rather high because 

 the absorption in such leaves is low. But at the violet end of 

 the spectrum as the yellow pigments come into operation and 

 absorb the radiations of short wave-lengths the absorption 

 becomes nearly as great as it is in green leaves, while the 

 transmission and reflection are greatly diminished. 



The properties of the leaf are modified by age. In the 

 region of the visible radiations, where these properties are 

 best known, the changes correspond to the enrichment in 

 chlorophyll of the young leaf, during which time the reflection 

 and transmission diminish, while the absorption increases. 

 They are halted during the adult period when the concentration 

 in chlorophyll is stationary. In the autumn, the destruction 

 of the chlorophyll causes an increase in the reflection factor 

 in the yellow and the red, while the yellow pigments, which 

 are always present, maintain the strong blue-violet absorption. 



These variations are slow, but there are also rapid variations 

 which take place under the influence of the fight itself; the 

 chloroplasts are displaced and oriented according to the 

 illumination so as to reduce the variations of the fight 

 absorbed. When it is weak, they arrange themselves in such a 

 way as to lose as little of it as possible; on the other hand, 

 when it is strong, they are prepared to absorb a much smaUer 

 proportion. 



The leaves of Tradescantia vindis (and also Pelargonium 

 zonale, Adiantum cuneatum, and Coleus hybridus) show this 

 phenomenon particularly clearly. For a period of the order 



