INTRODUCTION 



Without light, there is no vegetation. Light is one of the 

 physical factors which condition the life of the plant, but other 

 physical, as well as chemical, factors are also necessary. The 

 plant is a Uving organism in which chemical synthesis and 

 decomposition take place and these processes are possible 

 only in suitable chemical and physical conditions. 



Every variation in these conditions has its effect on the 

 behaviour of the plant to a greater or less degree; the various 

 factors react on one another and it is often difficult to isolate 

 the influence of one of them. A chemical action, for instance, 

 may be very different in different physical conditions and even 

 the distinction between physical and chemical is to some 

 extent arbitrary. To explain the known facts, however, a 

 distinction has to be made. 



The following chapters are concerned with the study of 

 one of the physical factors of plant Hfe, Ught, whose role is 

 fundamental. 



The vegetable organism could be compared to a chemical 

 factory, the gases in the atmosphere and the liquids and 

 sohds in the soil being its raw materials. But eyery factory 

 needs energy — ^in electrical, thermal, hydraulic or some other 

 form. The plant has the same need; the fundamental synthetic 

 processes by which it creates its own substance and the move- 

 ments of water and sap demand a continual supply. Whence 

 does it come? Light, luminous energy — nearly always from 

 the sun— provides the "motive power" of the vegetative 

 factory. 



The power, in the form of light, furnished by the sun 

 when it is at the zenith, can reach the astonishing figure of 

 3,200 kW for an area of 1 acre. In the Paris region, the solar 

 energy received per acre in a year is equivalent to an uninter- 



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