Prolonged artificial lengthening of the light period could prove 

 relatively expensive if electric current costs were high. Recently, 

 however, it has been shown that it is not necessary to supply illumi- 

 nation for the full length of time which would be required to provide 

 the equivalent of a long photoperiod. The photoperiodic effect of 

 interrupting the dark period for a few minutes to an hour near its 

 middle is similar to the effect produced when light is used continuously 

 to extend the natural photoperiod. Not only does this result in 

 reduction of the cost for light, but often the plants and flowers pro- 

 duced are commercially superior to those grown with extended periods 

 of artificial illumination. The length of the interruption and the 

 time of its application should be determined experimentally. 



Selection of Localities with Suitable Photoperiods 



Pyrethrum, a relative of the chrysanthemum, is used for the 

 manufacture of an insect powder. Large quantities are needed 

 annually. Since the insecticidal constituent occurs in the flowers, 

 it is desirable that plants produce a large number of flowers. It is 

 possible to grow this crop commercially in the United States, but 

 to do so care must be taken to select the proper localities. Pyrethrum 

 is a plant of the long-day type, and its period of blooming is also 

 dependent upon the prevalent temperature conditions. Selection 

 of suitable sites should take into consideration both the temperature 

 and light conditions occurring throughout the year. 



Hemp, too, is photoperiodically sensitive and, although this fact 

 is seldom realized or given the attention it deserves, the selection of 

 the locality in which to grow hemp should be a matter of definite 

 concern because of the influence exerted on both the amount and 

 quality of the fiber obtained. Hemp plants are of two sexes. The 

 males, which produce pollen, are generally somewhat smaller and 

 more slender than the females, which bear the seeds. Under con- 

 ditions of short photoperiod there is a tendency for relatively more 

 male plants to develop than under long photoperiods. Such an 

 increase in numbers might well result in a change in the quality and 

 quantity of fiber produced in various regions, depending on the 

 prevailing photoperiod of any region or on the season of the year 

 when the plants are grown. 



Seasonal leaf fall of many native trees and shrubs is as much a 

 response to the prevailing seasonal day length as to other environ- 

 mental factors. When attempts are made to cultivate such photo- 

 periodically limited species in other parts of the world, failure is as 

 often due to then lack of adjustment to light conditions as to other 

 environmental factors. In cities, leaves on branches of street trees 

 illuminated by electric lights often remain green much longer than 

 those not so exposed. 



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