484 RADIATION BIOLOGY 



(Borthwick and Parker, 1938; Knott, 1934; Withrow et al., 1943) and 

 tuber formation (Hamner and Long, 1939). 



Many studies of the part played by translocation of the stimulus in 

 the final expression of photoperiodic control have been reviewed by 

 Murneek (1948) and Cholodny (1939). These studies often made use of 

 branched or grafted plants, one part of which was subjected to photo- 

 periodic conditions favorable for flowering, whereas the other was under 

 unfavorable conditions. In these cases defoliation of the part under 

 unfavorable photoperiods, probably by inducing translocation to it, pro- 

 moted flowering. Translocation of the photoperiodic stimulus depends 

 on contact of living cells (Withrow and Withrow, 1943) and seems to 

 occur mainly in vascular tissue, as indicated by the work of Harder (1948) 

 on Kalanchoe. Severing a petiole of an effective leaf and separating the 

 parts by lens paper block the response (Withrow and Withrow, 1943). 



Photoperiodic control of reproduction in some mammals and birds is 

 associated with the eye and is mediated by the pituitary. If the eyes 

 are covered, photoperiodic conditions that are otherwise satisfactory for 

 reproduction produce no effect (Burger, 1949). Experiments of this 

 type, however, do not prove that the eye is the effective organ. Benoit 

 and Ott (1944) found in ducks covered except for the eyes that gonads 

 developed in response to supplementary light. Sexual response of ducks 

 to visible radiation after extirpation of the eyes and severing of the optic 

 nerve convinced Benoit and Ott that the pituitary was directly respon- 

 sive. Light-transmission tests on heads of ducks indicated that some 

 visible radiation probably penetrated to the pituitary through the region 

 of the eye. Bissonnette (1938), working with the ferret, considered that 

 the eye was effective in perception. One ferret with cataracts was not 

 responsive to Hght conditions adequate for production of oestrus in normal 

 females, and cutting the optic nerves of others delayed response. 



THE PERIOD INVOLVED 



Photoperiodic responses are determined by the length of the dark 

 period. The literature, particularly that before 1945 on plants and even 

 that current for animals, turns attention almost entirely to the light 

 period. Most of the experimental work makes use of 24-hr cycles, as is 

 natural for experimental convenience, and thus establishes the length of 

 both the light and the dark period. Garner and AUard (1920) in their 

 first paper demonstrated that Soja max var. Peking and Aster linariifolius 

 are short-day plants. They found, however, that darkening the plants 

 for 4 hr in the middle of the light period was without effect. They 

 stated in 1931: "Indeed the effect of midday darkening is much the same 

 as if the plants remained in the light for the whole day." 



