30 THE FLOWERING PROCESS 



increasing intensity causes an increased response up to a certain point, 

 and then the response stays the same (maximum) even though 

 intensity increases (see Avena curve in Fig. 3-3). The intensity level 

 beyond which no change in response occurs is called the saturation 

 intensity. In flowering we are concerned with the light absorbing 

 reaction which informs the plant by adjusting its metabolism, as to 

 whether it is day or night. This reaction may be studied by interrupt- 

 ing with light the dark period which promotes flowering of short-day 

 plants or inhibits flowering of long-day plants. If we allow a reason- 

 able exposure time such as 1 to 5 min, the saturation intensities for 

 these reactions are relatively low, especially compared to sunlight. 

 Even the intensities under rather heavy cloud cover (as low as 

 100 ft-c) will cause saturation in time intervals of less than a minute 

 (saturation is approached in about one minute at 1 % sunlight). Thus 

 the plant, during the day, "sees" much more light than it needs to 

 completely saturate its response capacity in flowering. During the 

 day, then, changes in light intensity due to clouds, are probably of 

 no importance to this part of the plant's life, but the question 

 immediately arises, at what time during dawn or dusk is the plant 

 responding as though it were day, and at what time is it responding 

 as though it were night ? 



4. Twilight and the Flowering Response to Day-length 



In experimental work, it is convenient to turn the lights on or off 

 so that the plant changes in a small fraction of a second from 

 conditions of saturating light to complete darkness or vice versa. 

 Obviously in nature the change from light to dark or back to light 

 is not so abrupt. The duration of twilight on clear days will depend 

 upon the latitude and the time of year, since at a given time the 

 elevation of the sun in relation to the horizon and the air mass through 

 which the rays pass will depend upon these factors. In addition many 

 local variables might influence twilight, such as elevation, nearby 

 mountains, atmospheric conditions, and of course weather. Thus 

 the clear sky curves shown in Fig. 3-4 are only representative. They 

 should be improved by new measurements to show the intensities 

 of light (using energy terms instead of foot candles) in the specific 

 wavelength regions known to be of importance in photoperiodism. 

 In Fig. 3-4 intensities are shown on an absolute and on a logarithmic 

 scale as functions of true elevation of the sun in relation to the 



