CHAPTER III 



PHOTOMETRY OF THE LEAF 



The fundamental role of light in the life of plants can often 

 be seen by the arrangement of their leaves spread out hori- 

 zontally as if to expose as much of their surface as possible. 

 Sometimes there may be too much Hght so that beans under 

 the blazing summer sun turn their leaves edgewise to offer 

 the minimum surface to its rays and allow the hght to pass 

 to the ground almost without causing any shade. Even the 

 internal structure of the leaf is adapted to the illumination 

 that it receives. 



Measurement of the Absorption, Reflection and Transmission 

 Factors 



Before studying the complex phenomena which occur in 

 the plant under the influence of hght — phenomena of which 

 we still know very httle considering the immense importance 

 that we suspect them to have — there is a prehminary question 

 which must be answered: How can we measure the fraction 

 of hght absorbed by the leaf, for only that fraction can have 

 any effect? It is a physical problem which is not easy to solve 

 completely, but we know the correct methods by which to 

 approach it. 



Unfortunately, many measurements have been made in 

 ill-considered conditions and have given results which cannot 

 be accepted without reserve. But we may take the opportunity, 

 as this is a matter on which we can speak with some certainty, 

 of explaining the physical relations between radiation and 

 the plant, the latter being more particularly represented by its 

 leaves since their surface is generahy greater than its other 

 parts. 



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