PHOTOPERIODISM 505 



were found to be of the long-night type. The inheritance ratios closely 

 approach 3 : 1 in both cases. 



Inheritance of photoperiodic response in Sorgh^im vulgare varieties is 

 also controlled by a single gene, Ma, which is modified in its expression 

 by two others, Ma^ and Maz (Quinby and Karper, 1945). The genes 

 Ma2 and il/as are dependent for their expression on the presence of 

 dominant Ma. Quinby and Karper found, in the F2 progeny of a cross 

 between the intermediate maturing SA5484 Dwarf Yellow milo variety 

 and the early-maturing Sooner milo, that 46 of 192 progeny were early- 

 maturing. Genetically the cross was Ma ma^ maz X ma ma2 mas. Early 

 maturing corresponds to flowering under short night lengths and is an 

 indeterminate expression for sorghum. Single-gene control of inheritance 

 was also observed for crosses between the long-night teosinte, Euchlaena 

 mexicana, and Zea mays, which is only weakly responsive to long nights 

 (Langham, 1940). 



Inheritance of photoperiodic response in short-night plants has also 

 been found to be controlled by a single gene where tested. Bremer (1931) 

 and Bremer and Grana (1935) found this to be the case for Lactuca sativa; 

 the crosses were between the short-night winter lettuces and the inde- 

 terminate summer varieties. In the F2 generation of a cross between a 

 winter variety, Maiking, and a summer salad variety. Deacon, 1092 of 

 1473 plants resembled the Maiking parent in flowering early in the sum- 

 mer on short nights. Similar results were reported for another short- 

 night plant, Epilohium hirsutum (Ross, 1942). 



In tobacco the indeterminate photoperiodic response is partially domi- 

 nant over the long-night response (Lang, 1948). This is also possibl}' 

 the case for teosinte (Langham, 1940) and for some varieties of Tagetes 

 ereda (Little et at., 1940) in which the long-night character is also reces- 

 sive. In sorghum varieties, however, the long-night character is domi- 

 nant. The short-night character of Lactuca sativa and Epilohium hir- 

 siitum is dominant over the indeterminate. 



The ecological variation due to night length observed for animals would 

 suggest that the photoperiodic factor is expressed. A work possibly bear- 

 ing on the subject is that of Hafez (1950) on variation in the sexual sea- 

 sons of three breeds of sheep. The breeding season of ewes originating in 

 Scotland was in the period when the nights were 11.5-12.5 hr or more in 

 duration. Night lengths during the breeding season for a breed origi- 

 nating in southern England varied from 10.5 to 12 hr and were shorter 

 still for breeds having merino blood. 



The photoperiodic responses of the several plants studied in detail thus 

 indicate that a single gene effects control. The factor and its alleles 

 could well differ more in a quantitative sense in their influence on response 

 than in qualitative distinction. This would be in harmony with the 

 scheme shown in Fig. 10-4, in which all plants are considered as basically 



