sealed with paper tape could be placed over the 

 plants to provide darkness. 



Experimental procedures can be facilitated and 

 made more exact if an electric time switch is available 

 to turn the lights on and off at any desired time. 



Demonstrations D-l through 1) 4 show some of 

 the effects of the relative lengths of day and night on 

 plant growth and reproduction. These demonstra- 

 tions tell us that: 



• Some plants flower on short days and long nights, 

 whereas others flower on long days and short 

 nights. 



• Dormancy of woody plants in the autumn is 

 brought about by short days. 



• Daylength controls tuber and bulb formation as 

 well as flowering and dormancy. 



Additional experiments can be designed to answer 

 and demonstrate many other aspects of the photo- 

 periodic control of flowering, bulbing, and dormancy. 

 For example, we might ask, what is the critical day- 

 length for short-day plants? What is the longest day 

 (shortest night) that will induce flowering of short- 

 day plants? What is the shortest day (longest night) 

 that will induce flowering of long-day plants? When 

 a long dark period is interrupted by a brief interval 

 of light, what is the minimum energy required to 

 keept short-day plants vegetative or to induce flower- 

 ing of long-day plants? When is the most efficient 

 time to give the interruption during the dark period? 

 Is the control of flowering operated through the same 

 red, far-red reversible photoreaction that controls 

 other plant responses? 



GENERAL CULTURAL HINTS 



Soil should be sterilized for the demonstrations 

 when it is used in germination tests or as a medium 

 in which to grow seedlings. Sterilizing the soil 

 destroys harmful insects, disease-producing organ- 

 isms, and weed seeds. Soil may be sterilized by dif- 

 ferent methods: (1) Place small lots of moist soil in 

 a shallow pan and bake for at least 1 hour at a tem- 

 perature of 215° F., then cool but do not use for at 

 least 2 weeks; (2) place soil in an autoclave or pres- 

 sure cooker and steam sterilize at 15 pounds' pressure 

 for at least ^ ■> hour, then allow to stand for a mini- 

 mum of 2weeks; (3) sprinkle l quart of formaldehyde 

 solution (1 pint 37 percent commercial formaldehyde 

 to 3 ;i | gallons water) on 1 square fool by 6 inches of 

 soil placed in a box or bushel basket, then water 

 liberally and completely cover with plastic or heavy 

 cloth for 48 hours, stirring frequently to hasten 

 escape of the formaldehyde gas, and allow 2 weeks 



before use of the soil. (CAUTION: \»> nol 

 planting as long as fumes are present, becau-' 

 aldehyde gas is an irritating poison to human- and ifi 

 toxic to plants./ 



Plants are usually grown in clay pots of '.',-, 3J/2-, or 

 4-inch diameter filled with sterilized soil. Before the 

 soil is put into the pot a piece of broken pot ifl placed 

 in the bottom to cover the hole so that the soil will 

 not plug it and prevent good drainage. Clean [tot- 

 should always be used. 



When pots are not available or are for some reason 

 objectionable, plastic cups, polyethylene free/: 

 containers, or even tin cans may be used. Before the 

 containers are filled with soil, they should have one 

 or more holes punched in the bottom. The holes are 

 covered with fiberglass matting or plastic window 

 screen. Good drainage is imperative for good plant 

 growth. 



In the experiments described, two 40-watt cool- 

 white fluorescent lamps are usually adequate, and 

 will be used unless another number is specified. The 

 lamps should be no nearer than 2 inches from the 

 plants, and usually no farther than about a foot. 

 The generally recommended temperature for most 

 experiments with fluorescent lamps is 70° to 80° F. 

 Maintain this temperature unless another is specified. 



Studies of the effect of light on plant growth and 

 flowering require that at certain times the plants be 

 placed in total darkness. A "dark chamber" is used 

 for this purpose. A light-tight box is generally best, 

 but a room may sometimes be suitable. A fight-tight 

 box must be constructed so that there is adequate air 

 exchange between the inside and outside to prevent 

 overheating. A satisfactory and proven method is to 

 construct a frame of wood and cover it with at leasl 

 two layers of black sateen cloth. An entrance or door 

 can be provided by making an overlapping flap. 



CAUTION: See that cords and connection- for 

 light chambers do not present a tire hazard. Be SUTl 

 that incandescent-filament lamps are not too close 

 to combustible material. 



GENERAL REFERENCES 



BORTHWICK, II. A. 



L947. DAI LENG ni \\n n oh i i;iv 



Science in Fanning. U.S. Depl Igl 

 Yearbook L943 17: 273 283 



BORTHWICK, 11. V 



1953 PHOTOPERIODISM : I'HV DARK - 

 now NIGHTS v\o LIGHTS 

 PI \\r GROWTH. Elect, on the Farm 

 Mag 26: 11 13 



