SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 95 



In summarizing, it may be said that lateral radiation of intact 

 coleoptiles of Avena causes marked reduction in the growth substance 

 that can be obtained from the tips. Slightly more growth substance 

 M^as recovered from the illuminated than from the shaded side. How- 

 ever, when excised tips of Avena and Zea are subjected to lateral 

 illumination, less growth substance can be recovered from the side 

 near the light. There is no indication of an increased growth- 

 substance content on the shaded side, since the amount obtained was 

 less than that of the control. 



INFLUENCE OF LIGHT UPON GROWTH SUBSTANCE " DIFFUSED " 

 INTO AGAR 



Growth substance was obtained in agar blocks of half the standard 

 size (Dolk and Thimann, 1932) by placing them in contact with the cut 

 surfaces of Nicotiana shoots and Zea coleoptile tips. Each of these 



Table 6. — Intluence of Mercury Arc Radiation on Growth Substance 

 Diffused into agar for 2 hours in darkness {Nicotiana tabacum and Zea mays) 



» 5 tests made. 



blocks was cut into two approximately equal parts ; then one part was 

 kept in darkness, while the other was irradiated for 120 minutes. Then 

 they were cut into small pieces, and the latter were applied to Avena 

 test stumps. The data (table 6) for September 12 and 14 in which 

 Nicotiana was used, indicate a remarkable inactivation of growth sub- 

 stance by the light treatment. With another brand of agar (probably 

 much cleaner) obtained from a different manufacturer, there was 

 less difference between the light-treated blocks and the controls, as 

 shown in the data for August 29. Also, the difference was less with 

 Zea growth substance than with that obtained from Nicotiana. In 

 all cases, however, less growth substance curvature was obtained with 

 the irradiated portions of the agar blocks than with the darkened 

 control portions. 



