PART I 



LIGHT AND VEGETATION 



To the memory of M. Georges Truffaut, 

 whose practical spirit was alUed to a taste 

 for pure research and whose penetrating 

 intuition astonished the scientists with 

 whom he loved to surround himself. 



I shall always remember with emotion 

 the pleasure of our conversations together 

 when we sketched out the plan of this book. 



Jean Tenien 



