684 



BIOLOGICAL EFFECTS OF RADIATION 



range in day length below the critical and the same is true of the long-day 

 group for day lengths in excess of the critical. It appears that there are 

 some plants, however, for which it is possible to have a day length too 

 long as well as one too short to induce flowering. Most investigators have 

 accepted this plan of classification, at least in principle, though in a few 



Fig. 1. — Contrasted day-length responses of long-day and short-day types. Upper, 

 Steironema cUiatum, (L.) Raf., a long-day plant; lower, Rumex sp., a short-day plant. Left 

 to right, the light exposures were: full summer day at Washington, D. C, of 14 to 15 hr. ; 

 darkened 10 a.m. to 2 p.m.; 12-hr. day; 10-hr. day; 8-hr. day; 5-hr. day. For the long-day 

 type, Steironema, the critical light period for flowering under the conditions lies between 

 12 and 14 hr. and with periods below the critical the plant tends to remain in the leaf 

 rosette stage; for the short-day type, Rumex, the critical period is about 11 hr. and with 

 longer day lengths the plant develops indeterminate vegetative stems. Midday darkening 

 produces no formative effect in either type of plant. 



instances plants have been classified into long-day or short-day types 

 on the basis of relative growth and production of dry matter, or contrast 

 in development not involving reproduction, in response to long and 

 short day. There has been some attendant confusion in the discussion 

 and in the interpretation of results. Schick (65) has recently proposed 



