MISCELLANEOUS PUBLICATIONS 1251, U.S. DEPT. OF AGRICULTURE 



adequately ventilated banks of lights will provide 

 some day-night temperature differential. 



The lighting for growth chambers is commonly 

 provided by a combination of fluorescent and 

 incandescent illumination, banks of which com- 

 monly occupy the entire ceiling. The quality of 

 the light from artificial lamps is not equal to 

 sunlight (14). However, a satisfactory light quality 

 may be achieved by using approximately 4 watts 

 of cool-white fluorescent illumination per watt 

 of incandescent illumination. The incandescent 

 lamps need not be larger than 60 watts, although 

 100- watt lamps are sometimes used. 



Control of day length is an essential part of 

 environmental regulation. Timeclocks that can 

 be set to turn the lights on or off at any 15-minute 

 interval are satisfactory for most purposes. Regu- 

 lation of light intensity is most commonly achieved 

 by a combination of varying the distance between 

 the plant bed and the light bank and by varying 

 the number of lamps lighted at any given time. 

 Most growth chambers contain a mechanism 

 for adjusting the plant bed height and several 

 timeclocks, each controlling a part of the lights. 

 Although illumination as low as 400 ft.-c. may be 

 desirable for some purposes, most plants will 

 grow satisfactorily in a white-walled chamber 

 under a measured illumination of 1.000 to 2,500 

 ft.-c. 



Many growth chambers can provide illumina- 

 tion considerably in excess of 2,500 ft.-c; how- 

 ever, the literature reveals that 1,600 to 2,000 

 ft.-c. is usually sufficient for vigorous growth. 

 Achieving maximum growth rates may require 

 higher illumination. 



The lamp bank and associated electrical ballasts 

 generate a considerable amount of heat and this 

 must be dissipated if temperature control is to 

 be achieved. One of the better growth-chamber 

 arrangements has the ballasts in a compartment 

 insulated from the plant growth space and the 

 lamp bank itself isolated from the plant growth 

 space by a transparent barrier. If open lamp 

 banks are used in a laboratory or classroom, 



adequate ventilation must be provided to pre- 

 vent excessive heat buildup on the plant bed 

 and in the room itself. 



Some degree of humidity control is commonly 

 provided in many growth chambers. Such cham- 

 bers have a humidity sensing apparatus, which 

 controls the operation of a refrigeration coil to 

 trap unwanted water vapor, and a steam genera- 

 tor, wet pad, or aerosal generator to increase mois- 

 ture in the air. In closed growth chambers without 

 humidity control, precautions must be taken to 

 insure adequate ventilation and thereby prevent 

 the excessive moisture buildup that results from 

 transpirational water loss from plants held in 

 an enclosed space. Increased humidity may 

 promote mildew development. If plants are 

 grown under a light bank in a laboratory or 

 classroom, humidity control is not practical 

 and normal watering of the plants should be 

 sufficient. A fresh-air change every 2 hours in 

 the chamber is desirable to prevent excessive 

 carbon dioxide buildup at night and a depletion 

 during the day. 



Since there is so much variety in growth cham- 

 bers depending on the manufacturer and on the 

 purpose of the chamber, the reader is referred to 

 manuals and specifications of the manufacturers. 

 Anyone planning to purchase new chambers 

 should consult with those who have had practical 

 experience with several types. 



Seed Germinators 



At the Fargo laboratory a temperature-con- 

 trolled incubator is used for seed germination. 

 Most seeds are germinated in the dark, although 

 some require light. If a dark incubator is not 

 available, any dark cabinet in which the temper- 

 ature remains fairly constant between 21° and 

 27° C. can be used, and reasonably uniform and 

 reproducible seed germination of the species 

 listed here (p. 17) will occur. Other species 

 having special germination requirements oc- 

 casionally may be encountered. 



IMPORTANT FACTORS FOR PLANT GROWTH 



Illumination and quality of light, photoperiod, 

 temperature, relative humidity, and nutrition 

 all interact to affect plant growth. If any one 



of these factors is less than optimum, poor plant 

 growth can result (4, 5, lb). 



Good plant growth usually can be obtained 



