742 BIOLOGICAL EFFECTS OF RADIATION 



tip to base. The first 0.25 mm. proved to be 40 times as sensitive as the 

 region 0.25 to 0.5 mm. This region was 6 times as sensitive as the 

 region 0.5 to 0.75 mm. The sensitivity continued decreasing until 

 beyond 2 mm. it was only 1/36,000 that of the tip. By excising 1, 2, and 

 4 mm. of the tip of coleoptiles, they found that the rate of growth was 

 diminished the greater the length of tip removed and also the higher the 

 intensity of light to which they were exposed. 



Koningsberger (54) showed that preliminary illumination at a low 

 intensity did not prevent a typical light-growth reaction at a higher 

 intensity; in fact, the reactions caused by the two different intensities 

 proceeded simultaneously, but the magnitudes of the maxima and minima 

 were dependent upon the higher intensity. By varying the exposure 

 intervals it was possible to produce curves in which the peaks of one 

 reaction corresponded to the troughs of another and thus produce the 

 typical interference phenomenon in the resulting growth curve. 



With a comprehensive series of experiments on the light-growth 

 reaction of Avena Dillewijn (29) presented experimental evidence confirm- 

 ing Blaauw's hypothesis as to the nature of phototropic movement, and 

 explaining through the light-growth reaction why the type of response 

 given varied with the intensity of the light. He showed that at low 

 intensities there was a long-period light-growth reaction, while at higher 

 intensities the reaction had a much shorter period, from trough to trough. 

 The light-growth reactions for different zones of the coleoptile tip 

 were also determined for different intensities. If the subapical regions 

 (0 to 2 mm.) of plants from darkness are exposed to continuous illumina- 

 tion, a decrease in growth rate ensues, and this is followed by a return 

 to the initial rate. Plants in continuous light when darkened showed 

 first an increase in growth rate, followed by a return to the original rate. 

 A short exposure to light caused first a decrease (the light response), 

 followed by an increase above the original rate (the dark response). 

 Dillewijn measured the changes in permeability of hypocotyls of Heli- 

 anthus caused by exposures to light. A wave-formed curve resulted 

 which resembled somewhat the short-period light-growth curve; however, 

 more experiments will be needed to establish the correlation between the 

 two. 



Erman (31) confirmed the findings of Vogt to the effect that immedi- 

 ately after exposure to light there was a decided stimulation in rate of 

 growth for the first 3 min. Generally the maximum rate of growth 

 occurred during the first minute of ilkimination. Even 1 sec. of exposure 

 was sufficient to cause a response. Erman also showed that the sensi- 

 tivity to light varied in different varieties of Avena. 



Blaauw (13) found a retardation in elongation of roots of Sinapis 

 dlha when exposed to intensities of 1500 meter-candles of continuous 

 illumination. Roots of Raj)hanus sativus gave no reaction even when 



