700 BIOLOGICAL EFFECTS OF RADIATION 



and the day-length factor. With soy beans Gamer and Allard (22) 

 found under the conditions of the experiment that differences in water 

 supply of the soil had no effect on time of flowering, though a condition of 

 comparative drought slightly hastened ripening of the seed. Gilbert (29) 

 found that photoperiodic response was delayed in soy beans by a com- 

 bination of relatively low temperature and high humidity, was delayed in 

 Cosmos by the reverse combination of higher temperature and lower 

 humidity, and was not affected by the ranges covered in salvia. Inter- 

 relationships of day length and the supply of essential nutrients will 

 be considered in the discussion of internal conditions of the plant in rela- 

 tion to the light period. 



EFFECTS OF ABNORMAL LIGHT PERIODS 



Following up the fact that midday darkening, which in effect divides 

 the daily illumination into two short light periods, fails to induce the 

 usual responses of short-day and long-day plants to a short day. Garner 

 and Allard (27) have studied the effects of abnormally long and short 

 alternations of light and darkness. Darkening in the middle of the day, 

 even for as long as 4 or 5 hr., does not hasten reproductive activity in 

 the short-day type nor does it ordinarily delay reproductive processes in 

 the long-day type. In these respects the effects are negative, the plants 

 behaving as when exposed to the full day. Growth and general nutrition 

 of the plant, however, may be adversely affected. When the plants were 

 completely darkened on alternate days during the summer months, thus 

 introducing a 48-hr. cycle of about 15 hr. of light and 33 hr. of darkness, 

 the general effect was that of a short day, though this effect was materially 

 weakened. Exposing plants alternately to the full summer day and a 

 10-hr. day length produced an effect intermediate between those of a 

 short day and a long day with respect to initiation of flowering. 



Plants of the long-day and short-day types were grown with short 

 alternations of equal light and darkness ranging from 6 hr. to as short as 

 5 sec, using high-intensity artificial illumination. In all cases the 

 effects on flowering were the same as those obtained with midday darken- 

 ing, that is, all long-day plants flowered readily while short-day plants 

 remained in the vegetative stage. The results are essentially those 

 obtained with a long-day or continuous illumination. On the other 

 hand, the differential effects of the various alternations on nutrition and 

 growth of the plants were striking. As the equal periods of light and 

 darkness were progressively shortened, there was increasing evidence of 

 malnutrition and retardation in growth which in most cases reached a 

 climax with the 1-min. intervals. With further shortening of the alter- 

 nations there was definite improvement in the condition of the plants 

 and with the intervals of 5 sec. the general nutrition and growth of the 

 plants appeared to be normal. The long-day and short-day plants were 



