,ej2 Journal of Agricultural Research vo1.xviii.no.ii 



It may be worthy of note that in the tests under controlled conditions 

 the height of the Biloxi plants under a 12-hour light exposure was practi- 

 cally the same as that of the latest field plantings shown in figure 3, while 

 that of the controls was about the same as that of the early field plantings. 



Since in many cases the length of the growing period was greatly cur- 

 tailed by the forcing action of reduced light exposure on reproduction, the 

 amount of growth was necessarily limited thereby in those plants having 

 a determinate type of inflorescence ; but, in addition, measurements made 

 when the blossoming stage of the forced plants had been reached show 

 that the rate of growth was greater as the length of the exposure to light 

 increased. The measurements of height recorded under the several tests 

 relate to the final heights attained by the plants. In the species tested 

 no exceptions to the foregoing principle were encountered; but it is 

 possible, of course, that other species will be found to act differently. 

 It has been demonstrated by a number of investigators that when many 

 green plants are transferred from light to darkness the immediate effect 

 is an acceleration in the rate of growth; and, conversely, the first effect of 

 exposure to light is a retarding of growth. These facts, however, bear on 

 necessary relation to the total effect on rate of growth over a considerable 

 period of time produced by differences in the relative length of night and 

 day. 



It remains to be pointed out that striking differences in sensitiveness 

 to decreased length of the daily exposure to light were observed in the 

 different species \mder investigation. Aside from considerable reductions 

 in the rate of growth and slight chlorosis, soybeans, tobacco, aster, and 

 some others showed no ill effects from the reduced length of illumination, 

 while Hibiscus was not able to make any appreciable growth with the 

 illumination period reduced to nine hours, and lettuce was much more 

 seriously affected, all individuals having perished without making any 

 material growth. 



I^ENGTH OF DAIIvY LIGHT EXPOSURE; IN RELATION TO SEXUAL REPRODUCTION 



While the rate of growth of the species tested was markedly affected 

 by change in the length of the daily illumination period, the effects on 

 blossoming and fruiting are particularly interesting and important. The 

 experiments with soybeans included four varieties which range from 

 early to very late in maturing under normal conditions when grown in 

 the latitude of Washington, D. C. Thus, for plantings in the field ex- 

 tending through the month of May the average number of days from 

 germination to blossoming was approximately 27, 56, 70, and 105, 

 respectively, for the Mandarin, Peking, Tokyo, and Biloxi, the last- 

 named showing no open blossoms till early September. Table III brings 

 out several important facts regarding the effects of reduced light exposure 

 on these four varieties. 



