PHOTOPERIODISM 685 



that for those plants in which tuber formation is of primary importance 

 this response to the Ught period rather than flower formation be used as 

 a basis for classification. This suggestion seems logical though possibly 

 not entirely free from objection. Redington and Schick in their reviews 

 have listed the long-day, short-day, and neutral or indeterminate plants, 

 as far as they have been reported in the literature. 



FLOWERING AND FRUITING RESPONSES 



Although the light period may affect sexual reproduction both 

 qualitatively and quantitatively, up to the present most work has been 

 directed toward the qualitative effect. While most short-day plants 

 flower readily through a wide range in day lengths below the critical and 

 most long-day plants flower under various day lengths in excess of the 

 critical, available evidence indicates that there is a fairly definite opti- 

 mum length of day for initiation as well as for completion of flowering and 

 fruiting processes. Generally speaking, however, the intervals or gaps 

 between the different light periods employed by investigators have been 

 too wide to permit of establishing the optimum light period for any par- 

 ticular plant. Garner and AUard found for certain varieties of soy beans 

 no significant difference in time required for flowering under a 12-hr. 

 day and a 5-hr. day, but this does not preclude the possibility that an 

 intermediate day length would somewhat further accelerate flowering. 

 With these varieties a day length of 13 hr. or longer produces decided 

 delay in flowering. For hemp, a short-day type, McPhee (48) obtained 

 maximum acceleration of flowering with a 7-hr. period of light. McClel- 

 land (46) found that for Tephrosia Candida a 12-hr. day is optimum for 

 flowering while the process is inhibited by both a 10-hr. and a 13.2-hr. 

 day. AUard (4) found an 18-hr. day to be optimum for Sedum telephium, 

 although longer day lengths were not tried. According to Smith (69) 

 continuous illumination produces maximum acceleration of development, 

 including flowering and fruiting, in barley, oats, rye, and English sword 

 pea. Further work is needed on this phase of the photoperiodic response 

 with a more complete schedule of Ught periods, providing relatively nar- 

 row intervals between the periods and with better control of conditions. 



Only very limited data are available as to the quantitative effect of 

 day length on flowering and fruiting, at least as regards optimal light 

 periods. In case of the soy beans mentioned above the absolute yield of 

 seeds per plant was approximately 9 times as great with a 13-hr. day as 

 with a 10-hr. day, but with the shorter light period the relative weight 

 of the seed as compared with the weight of the stalk was more than 

 twice that with the longer Ught period. A day length especially favor- 

 able for flowering also increased the weight of the individual seed. In 

 cultures exposed to regulated day lengths of 4, 6, 8, 10, and 14 or 16 hr. 

 Lubimenko and Szeglova (42) obtained maximum relative weight of fruits 



