XX 



PLANT GROWTH IN CONTINUOUS ILLUMINATION 



John M. Arthur 



Boyce Thompson Institute for Plant Research, Yonkers, N. Y. 



Plant growth in supplemental and continuous artificial light. Evidence of injury 

 from continuous sunlight. General conclusions. References. 



The literature dealing with plant growth under continuous illumina- 

 tion falls naturally into three main divisions according to the light sources 

 used. The divisions are: continuous natural sunlight, sunlight supple- 

 mented by artificial light for 12 hr. or longer each night, and continuous 

 artificial light. The earliest observations on the effect of continuous 

 light on plants were concerned entirely with plants growing in con- 

 tinuous sunlight in the Arctic regions. Observation and experimentation 

 in the other two main divisions obviously began only with the develop- 

 ment of high-intensity light sources which approached sunlight in 

 brilliancy and which could be maintained over a considerable period of 

 time. 



At an early date European travelers within the Arctic circle were 

 impressed with the rapid growth of plants during the long days as com- 

 pared with that of similar species growing farther south. In this con- 

 nection the observations of Linnaeus, in 1739, as quoted by Smith (27) 

 are of interest: "Slowness of growth corresponds to the length of summer 

 nearer the poles, however, not entirely; because toward the poles the 

 summer is shorter but also has longer days and the plants thrive and grow 

 by the heat of the sun. In Paris the summer is longer than in Lapland ; 

 therefore the plants ripen later in France than in Lapland, the length of 

 time being counted from the appearance of the shoots until they bear 

 ripe fruit. In Paris the cool nights are longer, during which time the 

 plants rest; wherefore they also need more days to ripen. In Lapland 

 there is so to speak no night during the summer, therefore plants can grow 

 both day and night. For example: In 1732 grain sown May 31st was 

 mown ripe July 28th, maturing in 58 days. Rye sown May 31st, 1732, 

 was mown August 5th, maturing in 66 days. This took place in LuleS., 

 Lapmarck, and could not have happened farther south." 



Schiibeler (25), in 1880, also observed the remarkable development of 

 plants under continuous sunlight during the short summers of the 

 Arctic regions. More recently Albright (1, 2) has called attention to the 

 rapid development of grain, potatoes, and various species of vegetable- 



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