METHODS OF EXPERIMENTATION WITH COCKLEBUR 81 



Time switches may control the light regime. Karl Hamner at the 

 University of California at Los Angeles has reached some sort of an 

 ultimate with a large number of modified file cabinets, kept in a 

 temperature controlled room, which is provided with artificial lights 

 of relatively high intensity. The file cabinets are modified by remov- 

 ing the upper drawers and replacing the front of the lower drawer so 

 that the cabinet is light tight when the drawer is shut. The drawer 

 is opened and closed by an electric motor controlled with a time switch. 

 When the drawer opens, the plants are below the lights, and when the 

 drawer shuts they are in the dark. With this arrangement Hamner 

 has been able to study rather complex cycles of light and darkness 

 (see Chapter 8), a tedious research problem if plants must be moved 

 by hand in and out of darkrooms. 



Often it is necessary to treat plants in the dark. This requires a 

 safelight which will have no effect upon the photoperiodism response. 

 Such a safelight should be of green color and very low intensity. To 

 obtain pure green light we surround a green bulb (either incandescent 

 or fluorescent) with two or three layers of green or blue-green cello- 

 phane. The late Robert Withrow (77) developed very special green 

 safelights for use at the Smithsonian Institution in Washington D.C. 



2. Light Interruption Studies 



Short-day plants are inhibited in their flowering by interrupting the 

 dark period with light, and long-day plants are promoted in their 

 flowering by this same procedure. In experiments designed to study 

 this effect the investigator needs a source of high intensity light with 

 accurate time control, in an area close to the darkroom, protected so 

 that moving the plants doesn't expose them to unwanted light. If the 

 light interruption lasts for more than a very short time, leaf tempera- 

 tures must be controlled by cooling. Withrow devised a refluxing 

 system for removing infra-red wavelengths. The light passes through 

 water causing it to heat and evaporate. This takes place within an 

 enclosed container, and evaporated water is condensed on cold pipes 

 and dripped back into the water. 



In many studies, light quality must also be controlled, and often 

 this may be done most conveniently by controlling the light source 

 and using simple colored filters consisting of plastic or cellophane. 

 In some installations, light is passed through diffraction gratings, 

 prisms, or interference filters. At the plant industry station at 



