CHAPTER XI 



PHOTOPERIODISM 



Definition of Photoperiodism 



Light, the principal nourishment of green plants, must be 

 given to them in sufficient quantity and not in excess. It must 

 be of suitable composition, without injurious ultra-violet and 

 without too much infra-red; natural dayhght is in general 

 well adapted to them. There remains a third factor which is 

 extremely important — the length of the day and night. 



The same quantity of Hght may be offered each day in a 

 number of different ways, either by strong illuminations for a 

 few hours followed by a long night, or by lower illuminations 

 spread over a longer "period" and followed by a short night. 

 The development of the same plant under these varying 

 conditions may be profoundly different. With a certain period 

 of dayUght, a plant may be incapable of producing either 

 buds, or flowers or fruit; one species of onion will not form a 

 bulb; another tuberous plant will remain without a tuber; 

 a tree may remain in leaf until the winter and be killed by the 

 frost, while the same plants, supplied with the same quantity 

 of Ught, on the same ground and at the same temperature, 

 but with suitable periods of daylight and darkness, will 

 flower, produce seeds, bulbs and tubers and resist the winter 

 frost. 



These curious consequences of daylength, which were 

 called "photoperiodism" by the two Americans, Garner and 

 Allard, who drew attention to them in 1920, are therefore of 

 considerable economic importance. For example, a species 

 adapted to the long summer days of northern climates may 

 be incapable of developing at a lower latitude, even if the 

 temperature is the same, because the summer days are shorter. 



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