138 LIGHT, VEGETATION AND CHLOROPHYLL 



Chrysanthemums may be made to flower earUer or later, by 

 artificially lengthening the day with electric Ught, or by 

 shortening it with opaque material covering the plants. 



A thorough knowledge of these phenomena would be 

 of great advantage in obtaining a better yield of crops. This 

 investigation is in progress, but will inevitably take a long 

 time for several reasons. 



Photoperiodism is still rather mysterious. It is strange, for 

 example, that a very low illumination, of 5 to 10 lux — 10,000 

 times lower than the maximum illumination from the sun — 

 given at nightfall for a few hours to lengthen the day, is 

 sufficient to produce a fundamental change in the develop- 

 ment of a plant. It cannot be said that chemical substances 

 elaborated by the plant, necessitating a luminous activation 

 for their synthesis, will be produced in sufficient quantity 

 only after a rather long period of dayhght, for then the 

 quantity of light would be important and not its duration. 



Perhaps certain slow syntheses are possible only when the 

 action of the Hght is sufficiently prolonged, perhaps others go 

 beyond the stage of their accomplishment to give other 

 combinations if darkness comes later. Such stringent demands 

 can be imposed only for the synthesis of compounds of a 

 complex structure, produced in very small quantities, as is 

 shown by the action of very low illuminations, and acting as 

 catalysts or organizers, rather hke the genes which transmit 

 all the hereditary characteristics in the single cell of the egg. 



The observations themselves, and their diversity and 

 complexity, make this necessary study difficult. Experiments 

 can give useful results only under two conditions: 



(1) The species and variety under investigation must be 

 specified, for the behaviour of similar varieties may be very 

 diff'erent ; 



(2) As always, not only the duration, the intensity and the 

 composition of the illumination must be stated, but other 

 circumstances as well, and particularly the temperature, 

 which plays an essential part in the photoperiodic action of 

 daylength. 



