CONTROL BY LIGHT 217 



oortli, 1956, 1957). Althougli tlie data leave something to be de- 

 sired, at least parth because of the many difficulties involved in 

 controlling yeast sporulation at all, there is surely something here. 

 Turning from yeasts, there are several reports in the literature that 

 red light is effective in inhibiting the spore germination of several 

 parasitic fungi, including the powdery mildew of wheat (Dillon- 

 Weston, 1932; Pratt, 1944). Certainlv both these phenomena merit 

 more detailed investigation than they have received. 



Photoperiodism has been known in animals almost as long as in 

 the higher plants. As in plants, it may control many aspects of re- 

 production and development, as well as adaptations to temperature 

 and other seasonal changes. The most detailed studies have been 

 done on birds, but reptiles, mammals, and insects as well as certain 

 other invertebrates have also been investigated (see Withrow, 1959b). 



Identification of the photoreceptor pigment here immediately 

 poses a problem already touched upon. Is photoreception ocular or 

 non-ocular? If ocular, does it depend on the visual pigments or on 

 others? This question has been studied most in birds, and there is 

 evidence that both ocular and non-ocular photoreception can bring 

 about a photoperiodic gonadal response. Crude action spectra in- 

 dicate that red light is by far the most effective if the eyes are 

 intact. However, this may be due largely to differential screening 

 by the tissues. When light is given directly to the hypothalamus, 

 yellow, green, and blue as well as red are all effective (see Farner, 

 1958, 1959). 



Probably in most vertebrates, photoperiodically active light enters 

 bv the eye, but certainly direct photoreception in some portion of 

 the brain is possible in forms such as birds and some reptiles, in 

 wliich the skull is highly fenestrated (Bartholomew, 1959). It is 

 also worth noting that although the photoperiodic responses of fishes 

 have not been studied specifically with regard to photoreception 

 (Harrington, 1959), blinded fish are known to show pigmentation 

 responses to changes in illumination (Sumner, 1940). Although 

 these responses are not always the same as those of normal individ- 

 uals, they again suggest the presence of a non-visual control by light, 

 and this might occur also in photoperiodism. 



A further piece of evidence for non-visual photoreception in pho- 

 toperiodism has been recently provided by de Wilde et al. ( 1959 ) 

 working with the Colorado beetle (Leptinotarsa). Diapause in this, 



