112 PHOTOCONTROL OF GROWTH 



walls are slits that can accommodate colored glass or plastic filter 

 sheets. Each of the cabinets is placed under a separate metal frame 

 on which the light sources are mounted. As light sources, various types 

 of monophosphor fluorescent tubes, sodium lamps, and incandescent 

 lamps are used, depending on the spectral region required. The 

 filters (one sheet on each side, one on top) provide additional purifi- 

 cation of the light, e.g., eliminating the blue, green, and yellow 

 mercury lines from red light. Colored glass sheets of the sizes required 

 are selected from specimens of the ordinary trade glass with the aid 

 of a hand spectroscope. The intensity level aimed at in the cabinets 

 is -^40,000 ergs/cm- sec <^ sphere, as measured with a spherical 

 radiation meter (Wassink and van der Scheer, 1951). This is equiva- 

 lent more or less to an intensity of 10,000 ergs/cm- sec as measured 

 in one direction with a flat light meter. 



Apart from high-intensity illumination in separate, relatively very 

 pure spectral regions, various combinations can be achieved. Thus, 

 for example, we used two types of cabinets supplying near-infrared 

 radiation, one with a red glass filter and incandescent lamps only, the 

 other with red fluorescent tubes and red glass on the sides, thus sup- 

 plying the same amount of the same red light as in the pure red light 

 cabinet, while, in addition, near infrared (mixed with far infrared) 

 is supplied from the top by incandescent lamps filtered by red + blue 

 glass. 



Our blue light cabinet has recently been improved. The blue glass 

 filter used previously in combination with blue fluorescent tubes trans- 

 mits a small amount of near infrared.- We now have a blue plastic 

 filter which, according to any measuring instrument as well as to the 

 naked eye, is completely opaque to far red and near-infrared radia- 

 tion. When this filter is used in combination with red glass, even the 

 direct sun cannot be seen. We have also mounted red glass with a red 

 filter on top of a blue cabinet to study antagonisms in long-term ex- 

 periments. The adaptability of the equipment to various problems is 

 obvious. 



2 We have explained (Wassink and Stolwijk, 1956) why we prefer to keep 

 this term for the radiation next to the visible region instead of using the more 

 recent expression, far red. Hillman and Galston (1957) followed our termi- 

 nology. 



