392 abstracts: botany 



BOTANY. — Efeci of the relative length oj day and night and other fac- 

 tors of the environment on growth and reproduction in plants. W. 

 W. Garner and H. A. Allard. Journ. Agric. Res. 18: 

 553-606. 1920. 



It is found that the relative length of the day to which the plant is 

 exposed is in many species a factor of great importance in growth and 

 development, especially as regards sexual reproduction. While early, 

 medium, late and very late maturing varieties of so3'beans required at 

 Washington about 26, 62, 73 and no days, respectively, to attain the 

 flowering stage when exposed to the full seasonal length of day of the 

 summer, all of these varieties flowered within 28 days when the daily 

 light exposure was reduced to 12 hours or less. Certain varieties of 

 tobacco, aster, ragweed, ttc, behaved similarly. On the other hand, 

 certain plants, as Raphanus, Hibiscus, and Mikania, flowered only 

 under a relatively long daily light exposure. By suitable control of 

 the daily light exposure certain annuals were forced to complete two 

 cycles of alternate vegetative and reproductive activity in a single 

 season. It was found, also, that flowering may be delayed more or 

 less indefinitely when the length of the exposure is unfavorable, and 

 this may result in a corresponding prolongation in the period of growth. 

 By use of tungsten filament electric lamps to increase the illumination 

 period of the short winter days the results obtained with several species 

 were similar to those obtained during the natural long days of summer, 

 i. e., the vegetative or the reproductive phases of development were 

 initiated or inhibited, depending on the specific requirements of the 

 plants used. With an illumination period favorable both to vegetative 

 and reproductive activity there was a marked tendency in several 

 species toward the "ever-blooming" or "ever-bearing" habit. In all 

 species studied the rate of growth (increase in height) was proportional 

 to the length of the daily period of illumination. Although the length 

 of the daily illumination period may exercise a controlling influence on 

 the attainment of the reproductive stage, it appears from experiments 

 detailed in this paper that differences in light intensity ranging from 

 full normal sunlight to less than a fourth of the normal do not greatly 

 affect this phase of plant development. Hence it is concluded that, 

 within the range indicated, the total quantity of solar radiation re- 

 ceived by the plant daily during the summer season is of little impor- 

 tance directly as regards attainment of the flowering stage. The term 

 photoperiodism is suggested to indicate the response of the organism to 

 the relative length of day and night. W. W. G. 



