LIGHT AND VEGETATION 53 



5,500 A; both decrease towards the edges of the spectrum, 

 particularly towards the violet and the ultra-violet. 



The opposite is true for the absorption factor, which is 

 deduced from the preceding factors; it is greater at the ends 

 of the visible spectrum than in the green region; it is par- 

 ticularly high on the side of the short wave-lengths, towards 

 the blue and the violet (Fig. I, 12). 



Amateur photographers who remember the time when 

 the plates used were not orthochromatic and were particularly 

 sensitive only to the short wave-lengths (blue and violet) 

 know that foliage always appeared very dark on the proofs 

 because of the small quantity of light reflected by the leaves 

 in that spectral region. 



On the contrary, photographs obtained with plates sen- 

 sitive to the infra-red and with an infra-red filter before the 

 object show very Hght fohage. This confirms the result of 

 measurements made at the Institute of Optics by quite a 

 different method. These show that the reflection factor, 

 wnich is small in the red, increases rapidly towards the 

 infra-red. 



In this spectral region at the beginning of the infra-red, 

 the pigments of the leaf, chlorophyll in particular, are trans- 

 parent; nor has parenchyma any more marked selective 

 absorption. The leaf therefore acts hke a diffuser and its 

 superficial layers disperse in all directions the hght that they 

 receive. About half is returned; that is the reflected part. 

 The other half is diffused towards the interior of the leaf; 

 part of this, depending on the thickness of the leaf, passes 

 through it. Here the optical phenomena are the simplest. 



The red and blue radiations that chlorophyll absorbs 

 strongly are for the most part absorbed by the green chloro- 

 plasts. The radiations of shorter wave-lengths, violet and 

 ultra-violet, are absorbed by yeUow substances, such as 

 carotene, xanthophyll, etc., wliich are extremely opaque to 

 them. 



The infra-red rays of longer wave-length are absorbed by 

 water and it is certain that for radiations of wave-length 



