Murneek — 47 — Research in Photoperiodism 



of some sort or in a ventilated darkroom, or by extending the photoperiod 

 with electric illumination. This can be done either in the greenhouse 

 throughout the year or outdoors during the growing season. Quite similar 

 procedure has been used by later investigators, excepting for variability in 

 kind of light employed, such as the use of incandescent bulbs, carbon arc, 

 fluorescent tubes, etc. 



The intensity of radiant energy given to plants to extend the length of 

 day has varied greatly, but commonly has been more than 100 f.c. As re- 

 gards photoperiodism, most plants seem to be sensitive to surprisingly weak 

 illumination, occasionally to as low as 0.1 f.c. (Withrow and Benedict, 

 1936). Light from as large electric lamps as 1000 watts, if too close, may 

 increase excessively the plant's temperature, requiring the interposition of 

 cooling equipment, such as a cell of running water. Details in the light- 

 ing procedure were introduced when some part of the plant was exposed to 

 one and another to a different light regime, as in studies of localization of 

 the effect. 



Steinberg (1931) and Garner (1936) have described and used to 

 some extent for photoperiodism studies elaborate equipment in the form of 

 air-conditioned cabinets, in which plants are grown entirely under electric 

 light, controlled temperature, humidity, air movement, etc. — -in other words, 

 an artificial climate. Went (1943) and Lewis and Went (1945) have 

 used air-conditioned greenhouses, with light, temperature and humidity 

 under control. For investigations on the relation of temperature to length 

 of day effects, the greenhouse temperature must be controlled either during 

 the dark or light periods or both (Thompson, 1933 ; Roberts and Struck- 

 meyer, 1938, 1939; Long, 1939, Borthwick et al., 1943), while for de- 

 tailed tests on parts of the plant, leaves, stems or growing points have re- 

 ceived special chilling either by means of air or water currents (Chroboc- 

 ZEK, 1937; Borthwick, 1943). 



In determination of the photoperiodic responses to light of certain 

 wave lengths, various filters have been employed (Withrow and With- 

 row, 1940; Went, 1944) but direct spectrographic light has also been tried 

 (Parker et al., 1945). 



Comparatively recently grafting procedures have come into vogue in 

 studies of translocation of the photoperiodic stimulus (Melchers, 1937; 

 Cajlachjan, 1937, 1938; Hamner and Bonner, 1938; Heinze et al., 

 1942 ; Stout, 1945). Undoubtedly other special techniques will be resorted 

 to in the future for detailed investigations of photoperiodism. 



Plants are being given photoperiodic treatments to a considerable extent 

 in modern horticultural practice, especially in connection with greenhouse 

 culture. In the light of our present knowledge it is possible not only to 

 bring introduced species and varieties into flowering or fruiting, but do it at 

 a specific time, either by shortening or lengthening the light period. The 

 timing for specific dates of bloom is a very important floricultural prob- 

 lem, particularly when it is done out of season. This can now be controlled 

 so definitely, by adjusting either the photoperiod or temperature, or both, 

 that recommendations are given by speciaHsts in the field and used in prac- 

 tice (Laurie and Poesch, 1932 ; Greene et al., 1932 ; Poesch and Laurie, 

 1935; Post, 1942). 



