508 RADIATION BIOLOGY 



periods and high temperatures. Dark periods just adequate for bulb- 

 ing at high temperatures (Heath, 1943) do not permit bulbing at lower 

 temperatures. 



In the case of warm-blooded animals, environmental temperature seems 

 to play a minor part in sexual reproduction. The striking response of 

 egg laying at temperatures far below 0°C on artificially lengthened days 

 in winter in some of the first experiments with j uncos (Rowan, 1926) and 

 later work with pheasants (Bissonnette and Csech, 1941) emphasized 

 dark period rather than temperature as a controlling factor. In other 

 animals such as aphids, interaction between temperature and dark period 

 is possibly more pronounced but still not controlling. For instance, in 

 the green apple aphid (Marcovitch, 1924) this might be a factor deter- 

 mining the number of parthenogenetic generations preceding migration 

 in the spring. 



Some attention has been turned to the effect of rate of change of day 

 length on sexual response. In fact, the general practice in experiments 

 with animals has been to simulate the seasonal change in day length 

 (Bissonnette, 1932a; Rowan, 1926). Photoperiodically sensitive plants 

 and animals readily responded to abrupt interruption of dark periods, 

 in so far as they have been tested, as well as to either abrupt or gradual 

 change in the length of the photoperiod used in experiments. The actual 

 way, however, in which the length of the dark period changes with season 

 and with latitude might have an important interaction on photoperiodic 

 response of both plants and animals. The behavior of soybean affords 

 an example. The long-night Biloxi variety when shifted abruptly from 

 8- to 16-hr dark periods may form as few as three or four flowers in the 

 terminal inflorescence. Plants that pass through the critical length of 

 dark period under natural conditions, however, might have several times 

 this number. This same behavior has been observed in other soybean 

 varieties grown in two latitudes, the more southern one having the greater 

 number of flowers in its racemes. The more gradual change in night 

 length in the southern latitudes results in a longer time spent in the criti- 

 cal photoperiodic region. Under these conditions the terminal meristem 

 is able to function over a longer period before the night lengths increase 

 sufficiently to prevent further differentiation. There is similarly a time 

 course in the sexual development of animals which depends upon the 

 intensity of photoperiodic stimulation. This has been studied somewhat 

 by Bissonnette, working with Sturnus vulgaris (Bissonnette, 1931), and 

 by Marshall (1950). The rate of change of day length has also been 

 mentioned as a possible factor interacting with photoperiod in its effect 

 on bird migration (Allard, 1928); however, Bissonnette (1936b) com- 

 mented that migration might result from cycles of the anterior lobe of 

 the pituitary. 



Light intensity during the photoperiod when adequate to give moder- 



