PHOTOl'ElilODlSM 681 



environmental conditions as between the test plants and the controls in 

 any particular series of experiments. 



It was found that for most plants the relative length of day and night 

 is a very important factor in growth and development, particularly in 

 its action in initiating sexual reproduction. On the other hand, the 

 duration of the vegetative stage in some species and varieties was not 

 materially affected by length of day within the range occurring in the 

 latitude of Washington, D. C. The more sensitive plants normally attain 

 the flowering and fruiting stages only when the length of day falls within 

 certain limits and consequently these stages of development ordinarily 

 are reached only during certain seasons of the year. Of these plants 

 some respond to a relatively long day while others respond to a short 

 day. Different varieties or strains within the species often show marked 

 contrast in their day-length requirements. With a day length which is 

 unfavorable for initiation of reproductive processes vegetative grow^th 

 may continue for a prolonged period, while a favorable light period may 

 induce very early flowering and fruiting. In this way certain varieties 

 or species become early- or late-maturing, depending on the length of 

 day to which they are exposed. 



Some of the relationships existing between annuals, biennials, and 

 perennials, as such, depend in large measure on responses to the seasonal 

 range in length of day. By regulation of the daily light period the annual 

 cycle of the plant's activities can be greatly shortened or it can be 

 lengthened almost indefinitely. Certain annuals were made to complete 

 two cycles of alternate vegetative and reproductive activity in a single 

 season. Likewise annuals showed some of the growth characteristics of 

 perennials when exposed to a suitable day length. One of the out- 

 standing results obtained is that reducing the number of hours of illumi- 

 nation by midday darkening, thus subjecting the plants to two periods of 

 illumination daily, is not effective in inducing early blossoming in plants 

 which readily respond to a single daily exposure of short duration. 

 Within rather wide limits the total quantity of solar radiation received 

 by the plant ordinarily is not a determining factor as regards attainment 

 of the reproductive stage, that is, equivalent changes in the two factors, 

 intensity and duration of light, do not necessarily produce similar effects. 



In the species studied, including those in which flowering is induced 

 by a short day, those flowering in response to a long day and those in 

 which time of flowering was not affected by day length, the rate of growth 

 as measured by height attained increased with increase in length of day. 

 The daily light period is believed to be an important factor in the natural 

 distribution of plants and in crop-plant adaptation. 



The term photoperiodism was proposed to designate the phenomena 

 embraced in the response of the organism to the relative length of day 

 and night. 



