12 INTRODUCTION 



rupted average power of 560 kW. Thus, if the area of culti- 

 vated land, forests and pastures in France is estimated at 

 50,000,000 acres, it can be calculated that the average power 

 received by the vegetation in that area is 35,000,000,000 kW. 

 This power suppUes an annual quantity of energy which 

 is some thousands of times greater than the consumption of 

 electricity in France in 1954.^ 



Such an enormous quantity of luminous energy would be 

 sufficient to prove the fundamental importance of this 

 physical factor. Certainly, the plant absorbs for itself only a 

 small part of what is thus offered to it. Later we shall study 

 in detail what fraction is really utilized and the different 

 functions that it performs. We shall see that their multiplicity 

 also forces us to consider the physical factor of light as being 

 of supreme importance to vegetative growth. 



The truly scientific and precise study of the influence of 

 light began only a comparatively short time ago, but very 

 important results have already been obtained. Certain par- 

 ticularly striking observations deserve to be singled out in 

 this introduction. 



One of the first of these observations was the surprising 

 rapidity of the growth of vegetation in the far north near the 

 Arctic Circle where, during the summer, there is scarcely any 

 night. 



According to Linneus (in 1732) wheat sown on 31st May 

 was ripe and ready for cutting on 28th July, fifty-eight days 

 later, while the rye crop was gathered sixty-six days after 

 sowing. Only the long hours of daylight can explain this 

 phenomenal growth. At Lulea, on the Gulf of Bothnia 

 (lat. 65° 32' N), where these observations were made, the sun 

 scarcely disappears below the horizon at this time of the 

 year so that the insolation is almost continuous. 



Among the manifold effects of hght — effects of which the 

 importance and variety are becoming increasingly clear to us — 



'The consumption of electricity in France in 1954 was 45,000,000,000 

 kWh, which is equivalent to an uninterrupted average power of 

 5,100,000 kW. 



