Murneek — 43 — Research in Photoperiodism 



fected by light duration, it has been adopted by most investigators and is 

 used widely. 



In short-day plants flowering is induced experimentally by relatively 

 short photoperiods, usually 10 hours or less ; in long-day plants by photo- 

 periods of 14 hours or more; and in so-called "neutral" plants by 10-18 

 hours of light duration or even continuous illumination (Harvey, 1922). 

 Examples of short-day plants are: Some tobaccos and soybeans. Chrysan- 

 themum, Salvia, Poinsettia, Cosmos and many other of our spring and fall 

 flowers. Of long-day plants there are : Spring varieties of cereals, spinach, 

 lettuce, radish, potato, Rudbeckia and other summer flowers. Of neutral 

 plants we have : Tomato, dandelion, buckwheat, cotton and some squashes 

 and cucumbers. 



Recently Allard (1938) and Garner (1940) have added a fourth 

 group, of which there seem to be a few representatives, designating it 

 intermediates. They seem to flower at a day length of intermediate dura- 

 tion (12-14 hours), but are inhibited in reproduction by day lengths either 

 above or below this length. Examples of such plants are : Mikania scandens, 

 Phaseolus polystachyns, Eupatorium torreyanum and Saccharum spon- 

 taneum. 



To lengthen the light period experimentally. Garner and Allard 

 found electric light of comparatively low intensity consistently effective for 

 initiation or inhibition of reproduction or vegetation, depending on the type 

 of plant treated. This has been fully verified by other investigators and is 

 good evidence that, in general, intensity of the supplementary light used 

 is not a factor in photoperiodism. Very weak light, of course, cannot be 

 used, throughout the photoperiod for a large number of days without great 

 disturbance in growth and development of plants. 



Photoperiodism is an important factor in the natural distribution of 

 plants. In their native habitats plants are adapted in various degrees to 

 a variety of environmental factors, including the diurnal duration of light. 

 They could not persist long in a specific region or extend their range unless 

 the environment is favorable for some form of reproduction (Potapenko, 

 1945). Numerous observations have shown that genera, species and va- 

 rieties have developed photoperiodic responses which enable them to adjust 

 the time of flowering and fruiting (seed production) to definite seasons, 

 characterized by certain lengths of day. Thus we have more or less typical 

 spring, summer and fall blooming types of plants. The literature on this 

 subject is voluminous, of which the following papers may be cited as exam- 

 ples: Adams, 1923; Allard, 1932; Darrow, 1934; Doroshenko, 1927; 

 DoROSHENKO and Rasumov, 1929 ; McClelland, 1928 ; Maximov, 1929 ; 

 Tincker, 1925; Kuznetsova, 1929; Lubimenko and Sceglova, 1927; 

 Allard and Zaumeyer, 1944; Allard and Garner, 1940 and Bunning, 

 1943. Some synchronization to the seasonal photoperiod is remarkably 

 close (Smith, 1941). 



In general, plants that have originated in southern or tropical latitudes 

 will require short days for flowering, while those of more northern lati- 

 tudes, roughly to the north of 60°, are long-day plants. When the latter are 

 moved too far south, they will not produce blossoms. When carried farther 

 north, they will still be reproductive, because of an increased photoperiod 



