EFFECTS OF LIGHT INTENSITY 741 



their total lengths were correspondingly less, being 84, 80, 75, and 62, 

 respectively, based on their growth in darkness as 100. Measuring the 

 growth rate every 3 min. he found that exposures to intensities less than 

 1500 meter-candles for periods up to 15 min. caused first a depression in 

 growth, followed by an acceleration; whereas exposures to intensities of 

 12,000 to 400,000 meter-candles caused an immediate increase in growth 

 rate during the first 3 min. of exposure, followed by a decrease below the 

 original rate. He found the same responses reported by Blaauw for 

 exposures to continuous light. Preliminary illumination at 5 meter- 

 candles did not change the course of the response to higher intensities. 

 Plants kept in light and exposed to a short period of darkness showed a 

 stimulation in rate of growth. Sudden changes in temperature also 

 caused the growth curve to exhibit an undulating character. 



Sierp (97) confirmed Vogt's work that the Avena coleoptile con- 

 tinued growing longest in darkness and that the length of the growth 

 period and total length of the coleoptile decreased with increasing 

 light intensity. This held true for plants illuminated from the start 

 and also for those given illumination after having grown for different 

 lengths of time in darkness. In his experiments he measured the rate 

 of growth only at intervals of 1 hr. or more; hence he did not detect the 

 variations occurring in shorter intervals. The response he noted was 

 not the typical light-growth reaction of Vogt and Blaauw. 



In a later paper, Sierp (98) measured the growth rate at shorter 

 intervals and was able to observe the typical light-growth reaction when 

 the coleoptiles were exposed to only 100 meter-candle-seconds, 2 sec. at 

 50 meter-candles, or 10 sec. at 10 meter-candles. Weak responses 

 occurred to light as low as 10 meter-candles for 1 sec. 



This wave-formed growth curve was produced not only by exposures 

 to light, but also by exposing the plants to the narcotic action of ether, or 

 to a shock (99). While exposed to vapors of ether, the plants were 

 still able to give a typical light-growth response when exposed to light. 

 When etiolated coleoptiles were exposed to less than 100 meter-candle- 

 seconds they tended to show first a maximum at 40 min. and a minimum 

 later at 100 min. As the amount of hght was increased to 3000 meter- 

 candle-seconds, the light-growth reaction changed so that the minimum 

 occurred first at 30 min. and the first maximum at 70 min. The peaks 

 and depths of the curve increased with increasing amounts of light. 

 When the growing tips w^ere covered with tinfoil the typical light-growth 

 reaction was pronouncedly disturbed, and frequently did not occur at all, 

 whereas darkening the growing region tended only to delay the beginning 

 of the reaction. 



By covering the growing region with tinfoil bands and caps, Sierp and 

 Seybold (100) showed the light sensitivity, as measured by the time 

 required for the initiation of phototropic bending, to vary greatly from 



