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1920] CURRENT LITERATURE 247 



varied in the different tests from 5 hours daily to full daylight, 7 and 12 hours 

 being the exposures chiefly used. Checks received full daylight under similar 

 conditions of temperature. Shorter light exposures were all made during the 

 middle of the day, and during the time of highest light intensity, except one 

 series of soy beans which were kept in darkness from 10:00 a.m. to 2:00 p.m. 

 daily. 



t 



In general, the amount of vegetative growth was proportional to the 

 length of daily exposure to light. The short exposures resulted in short, 

 slender plants of greatly reduced size. Rate of growth was much slower, and 

 the total size attained was reduced. The inception of the flowering or 

 reproductive phase was greatly influenced by length of exposure to light. 

 Many of the species worked with w T ere thrown into flowering and fruiting by 

 the shorter exposures, while with certain other species and varieties, reducing 

 the period of illumination had little effect upon the inception of fruiting. 



The authors conclude that for each plant there is a "critical" length of 

 daylight exposure essential to the development of the fruiting phase. The 

 length of this critical exposure varies w 7 ith each species and variety, but, in 

 many individuals at least, is very much shorter than normal summer daylight. 

 By exposing the plants to this critical length of illumination, the reproductive 

 or flowering phase can be induced at almost any time. By varying this time 

 of exposure, typical biennials, as Aster linariifolius, could be made to com- 

 plete their life cycles within a few months, w T hile annuals, as soy beans, Solicfago, 

 etc., could be induced to respond as biennials. 



Experiments with shading indicated that time of exposure, and not light 

 intensity, is the primary factor involved in determining the critical day. 

 Light intensity reduced to 43 per cent by shading had no effect upon the time of 

 inception of fruiting, although it did give typical shading results on form and 

 amount of growth. Of significance, however, is the result obtained from 

 exposing soy bean morning and afternoon, but keeping it in darkness during 

 midday. Time of fruiting was not materially altered by this treatment, 

 although it was much advanced in the same variety by reducing the exposure 

 to light through leaving in darkness morning and evening. Reducing the 

 water supply reduced vegetative growth and fruit yield, but did not alter time 

 of fruiting in the least. Winter light, supplemented by artificial illumination 

 at night, giving a total daily exposure of 18 hours, acted exactly as long sum- 

 mer daylight in its tendency to retard or prevent fruiting. The authors 

 believe length of day, through its influence on fruiting and seed formation, to 

 be a fundamental factor in plant distribution. 



No attempt has been made by the authors to explain how length of day 

 might thus determine the form of plant development. It is unfortunate 

 that a more careful review of the literature was not given, as the authors have 

 made no attempt to link their work to other very critical studies along this 

 line. Klebs found that by varying the salt nutrients, he could induce 

 vegetative or reproductive growth at will, over a very wide range of plants. 



