ous-rooted begonia are examples of long-day plants, 

 which flower in nature only when the days are long 

 and the nights are short. Flowering of many other 

 kinds of plants is hastened but not absolutely con- 

 trolled by the appropriate daylength. For example, 

 scarlet sage, variety America, flowers quickly on 

 short days but eventually flowers on long ones. Many 

 varieties of petunia flower most rapidly on long days 

 but finally flower on daylengths as short as 8 hours. 



Bulbing and tuber formation are also controlled by 

 daylength. Tuberous-rooted begonia, which is a long- 

 day plant for flowering, produces tubers on short 

 days but not on long days. Onions, on the other 

 hand, produce bulbs on long days but not when the 

 days are short. 



Dormancy, and thereby preparation of woody 

 plants for the coming of winter, is another plant 

 response regulated by photoperiod. Even in the warm 

 greenhouse many woody plants stop elongation of 

 stems, produce terminal buds, and "harden off" when 

 the days begin to shorten in the autumn. However, if 

 artificial light is used to keep the days long, plants 

 in the warm greenhouse will continue growing during 

 the naturally short days of winter and several years' 

 "field" growth is often obtained in only 1 year. 



These plant responses are regulated not by the 

 length of the light period but by the length of the 

 dark period. Thus, a long-day plant is really a short- 

 night plant, and a short-day plant is really a long- 

 night plant. Therefore, when a long dark period is 

 broken into two short periods by a relatively brief 

 exposure to light near the middle of the period, long- 

 day plants bloom, dormancy of woody plants is pre- 

 vented, and onions produce bulbs. Under these same 

 conditions, short-day plants remain vegetative. 



Physiological studies often require that plants be 

 grown indoors, with temperature controlled and 

 periods of light and dark regulated. The plants, how- 

 ever, should have the same healthy appearance as 

 well-tended plants grown out of doors. Everyone 

 knows that plants cannot survive without light of 

 adequate intensity to operate the processes of photo- 

 synthesis. In the field and garden or in the green- 

 house this high-intensity light is obtained from the 

 sun, which often provides an illumination as high as 

 10,000 foot-candles. In the average home the light 

 intensity is usually too low for growth of many kinds 

 of plants, even on the window sills. However, plants 

 can be grown quite successfully with artificial light 

 in complete absence of sunlight. Beans, tomato, 

 cereals, and many ornamentals that grow in open 

 sunlight make satisfactory growth if the artificial 

 light intensity is about 1,000 foot-candles. Shade-lov- 



ing plants, such as African-violets, begonias, episcias, 

 gloxinias, and orchids, will grow well with intensities 

 as low as 500 foot-candles. 



A practical source of artificial light for plant 

 growth is the fluorescent lamp. These lamps supply 

 the necessary intensity without excessive heat and 

 are available in various lengths, wattages, and colors. 

 They are usually operated on one- or two-lamp bal- 

 lasts, which maintain the proper current and provide 

 the starting voltage. Prewired lamps and ballasts of 

 several sizes and types are available as commercial 

 luminaires or as channels. 



Many kinds of plants can be grown satisfactorily 

 with only two 40-watt fluorescent lamps. As the 

 lamps themselves are relatively cool, the plants may 

 be placed quite close to them without danger of ex- 

 cessive heat or burning. Table 1 shows the illumina- 

 tion at various distances from two 40-watt cool-white 

 fluorescent lamps mounted 2 inches apart. If the 

 lamps are mounted further apart, the illumination at 

 6 inches or less from the lamps is markedly decreased. 



Table 1. — Illumination in foot-candles at various dis- 

 tances from two or four 40-watt standard cool-white 

 fluorescent lamps mounted approximately 2 inches 

 from a white-painted reflecting surface 





Illumination 



Distance 







from lamps 





Four lamps 1 



(inches) 



Two lamps, 1 

 Used 2 















Used 2 



New 





Ft.-c. 



Ft.-c. 



Ft.-c. 



1 



1,100 



1,600 



1,800 



2 



860 



1,400 



1,600 



3 



680 



1,300 



1,400 



4 



570 



1,100 



1,300 



5 



500 



940 



1,150 



6 



420 



820 



1,000 



7 



360 



720 



900 



8 



330 



660 



830 



9 



300 



600 



780 



10 



280 



560 



720 



11 



260 



510 



660 



12 



240 



480 



600 



18 



130 



320 



420 



24 



100 



190 



260 



1 Center-to-center distance between the lamps was 2 inches. 



2 These lamps had been used for approximately 200 hours. 



If the daylength is to be controlled, plants must be 

 put into complete darkness at the close of a particular 

 photoperiod. A dark chamber can be made of 

 Masonite or plywood with calked seams, or it could 

 be made of two or more thicknesses of black sateen 

 cloth stretched over a wooden frame. If used care- 

 fully, a cardboard box with all seams and joints 



