RADIANT ENERGY 405 



have made the interesting discovery that certainly the time of 

 flowering if not the actual flowering of plants is, within limits, 

 determined by the length of daylight to which the plants are 

 exposed. They were able, by controlling the time of exposure of 

 a plant to light, greatly to decrease or increase the age at which 

 the plant reaches sexual maturity. Thus, the field aster, which 

 commonly requires four months (May to September) to reach 

 sexual maturity, was made, by decreasing the time of exposure to 

 daylight, to bear flowers within a month after germination (by 

 June 18). Still more remarkable is the fact that these same 

 plants, instead of completing their life cycle by dying after 

 flowering, as they would have done in the field, developed new 

 axfllary branches (on being restored to normal light exposure) 

 and flowered a second time in September. 



It is thus evident that certain characters of a deep-seated and 

 fundamental nature, which heretofore have been regarded as 

 immutable, are relatively unstable and respond readily to change 

 in the external environment. It is, consequently, not surprising 

 for some biologists to hold that all "characters are of the nature 

 of responses to environment" and that "every life process must 

 to some degree be dependent upon the external world." 



These studies assume a practical significance in connection 

 with the growing of plants in North Russia and Alaska where the 

 season is short and daylight hmited. If plants can be forced 

 into flower and fruit more quickly by shorter or longer exposure 

 to fight, then it will be possible to bring fresh fruit and vegetables 

 on to the markets at a time that under normal conditions would 

 not be possible. 



The question of the relative significance of the different parts 

 of the spectrum on photosynthetic activity has long existed 

 among botanists. The importance of ultraviolet light was at 

 first emphasized, until it was realized that plants do well in green- 

 houses the glass of which permits little or no ultraviolet to enter. 

 Emphasis was then laid on the infrared. It now appears that 

 plants can use both ends of the spectrum — that while the blue- 

 violet (and ultraviolet) end of the spectrum is not indispensable, 

 it is necessary for vigorous growth. Absence of all wave lengths 

 shorter than 529 m/i results in a condition similar to that in 

 which plants are grown in very subdued fight; infrared-illumi- 

 nated plants are larger but less green. The infrared is, therefore. 



