NO. 20 GROWTH SUBSTANCE— BURKHOI-DER AND JOHNSTON 7 



stated, the agar blocks were applied unilaterally to Avcna stumps. 

 The curvatures obtained in this way were markedly reduced by light 

 treatment (table 3). 



Similar reduction in active growth substance was obtained by 

 exposing to light excised tips placed on glass slides in moist petri 

 dishes (table 3). Whether the whole tips or split half tips were 

 applied (with a little gelatin) to Avena stumps, light reduced the 

 curvature to less than half that given by tips maintained in darkness. 

 Furthermore, an exposure of 240 minutes produced- still greater loss 

 of growth substance than did the exposures lasting for 120 minutes. 



In ex^^eriments on September 14, one set of 12 excised tips was 

 illuminated for 120 minutes, then kept in darkness 120 minutes, 

 another set of 12 was kept darkened 120 minutes, then illuminated 

 120 minutes, and a third set of 12 was illuminated 240 minutes. 

 About the same small curvature (2.1 and 2.5 degrees) was obtained 

 with 120 minutes of radiation irrespective of whether 120 minutes 

 of darkness came before or after the light treatment. On September 

 17, 12 tips irradiated for 120 minutes and directly applied to test 

 plants yielded 2.1 degrees curvature, while the dark controls gave 13. i. 

 In the experiments of September 14, 240 minutes of irradiation 

 reduced the growth substance to zero. 



In summarizing, it would appear that radiation above certain 

 minimum quantities in the ultraviolet and visible regions of the 

 mercury arc spectrum causes a decrease in amount of growth sub- 

 stance that can be obtained from intact and excised coleoptile tips of 

 Avcna and Zca. 



INFLUENCE OF UNILATERAL RADIATION UPON GROWTH SUBSTANCE 

 IN COLEOPTILE TIPS 



Avcna sativa.- — Intact coleoptiles were irradiated from one side for 

 various lengths of time. During this irradiation the plants were 

 oriented in specially constructed glass holders so that the direction 

 of the light beam was through the longer transverse axis — that is, 

 one of the narrow edges of the coleoptile was illuminated. Curvatures 

 occurred toward the light. At the end of the period of light exposure 

 the tips were excised and split vertically into halves which then 

 were applied unilaterally with gelatin to Avena stumps. In all cases 

 the half tips near the light produced slightly greater curvatures than 

 did those on the shaded side (table 4). This same table also indicates 

 that lateral light does not cause increased accumulation of growth sub- 

 stance below the tip when the amount obtained from the tip is 

 lowered. 



