INTRODUCTION 15 



The greater part of our planet is covered by the oceans; 

 there, also, solar illumination produces crops with the help of 

 the microscopic algae which float between the depths of 100 

 and 150 metres; in the Channel, the yield is 5-6 tons per acre 

 of vegetable plankton. The algae of this plankton are chloro- 

 phyll plants, and the catch of certain fish which feed on them 

 can be estimated according to the number of hours of sun- 

 shine at the beginning of spring. In Scotland, fertiUzers 

 poured into some almost enclosed lochs have produced a 

 notable increase in plankton and a more rapid development 

 of the fish living in these cultivated marine fields. 



Photosynthesis is only one aspect of the action of light 

 on vegetation. In the following chapters we shall give some 

 details of other influences and a ghmpse of the immense 

 discoveries which are still possible. But first it is necessary to 

 define what light is and to state the facts which, in the 

 knowledge acquired by physics of the nature of radiation, will 

 enable us to understand a small part of the still mysterious 

 effects of light on the life of the plant. 



