Artificial Light 



A basic fact about indoor light installations is that the two types of 

 commonly used lamps differ greatly in the proportions of red and far red 

 in their white light. Light from ordinary incandescent filament lamps 

 contains large amounts of both red and far red, as sunlight does. Light 

 fronn fluorescent lamps is high in red and extremely low in far red. In a 

 greenhouse with fluorescent installations, if more far red is wanted, it 

 can be obtained by adding some incandescent lamps, since plants sense 

 and respond to such adjustnrients. 



Both incandescent and fluorescent lamps have a useful place in 

 artificially lighted growth rooms. For example, fluorescent installations 

 are the better source for fulfilling high-intensity requirements. On the 

 other hand, incandescent lamps have proved more efficient for purposes 

 such as speeding the flowering of long-day plants and promoting swifter 

 growth in woody and foliage plants --even though these lamps shed much 

 of the inhibiting far red. 



When separate red or far -red light is specifically wanted for indoor 

 plant work, filters of colored cellophpine can provide relatively pure 

 color. Almost pure red can be obtained by equipping a fluorescent lamp 

 with a red filter, because this blocks out light waves shorter than those 

 of the red band, and the lamp itself produces a negligible amount of far 

 red. Amost pure far red can be obtained by equipping an incandescent 

 filament lamp with blue and red filters, because these together block out 

 almost the entire visible spectrum, letting through the far red which 

 such lamps can provide. 



THE ROAD AHEAD 



Pioneering plant scientists have gone far in exploring light-growth 

 relationships, but still regard the interior of growing plants as a dark 

 continent to a considerable extent. The intricacy of photoperiodic reactions 

 is evident, even in a brief account such as this, limited to major landmark 

 discoveries with a little experimental work cited. Actually, experiments 

 by hundreds have contributed facts that clarify and complicate light-growth 

 processes. World-wide scientific reports on photoperiodism are nu- 

 merous. 



Photoperiodic specialists have predicted that discoveries in the 

 coming decade should outstrip past progress. From today's position, 

 scientific research can work forward toward further goals, such as 

 completely analyzing and describing the chennical structure of phytochrome 

 and establishing the nature of its enzynnatic action. Because phytochrome 

 is a protein, it probably cannot be made synthetically but its enzymatic 

 action probably can be controlled by compounds still to be discovered. 



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