NO. 20 GROWTH SUBSTANCE BURKHOLDER AND JOHNSTON 5 



separate series showed a curvature of 9.8 degrees produced by control 

 tips, while illuminated tips yielded only 3.1 degrees. Ultraviolet 

 irradiation for 120 minutes reduced the curvature to i.i degrees. 

 Another experiment performed August 27 with light from the total 

 arc indicated that the normal quantity of growth substance contained 

 in the tips of darkened plants is not driven downward into lower 

 levels of the coleoptile during radiation. At a level 1.5 cm below the 

 tip there was no increase in growth substance commensurate with the 

 marked decrease at the tip caused by illumination. 



A series of experiments was performed to determine whether illumi- 

 nation might reduce the amount of growth substance contained in 

 excised coleoptile tips. The tips were excised with a razor blade, and 

 after being moistened on the cut surface with a small amount of 5 

 percent gelatin, they were placed upon a clean glass plate in a petri 

 dish lined with wet filter paper to prevent desiccation. Tips were 

 irradiated in these dishes for different lengths of time, and excised 

 control tips of each series were allowed to stand in darkness for the 

 same time as the exposed tips (table 2). Irradiation for 60 or 120 

 minutes clearly reduced the amount of curvature obtained in the tests 

 with Avena stumps. Seven series of experiments with both Victory 

 and Markton varieties of oats treated for 120 minutes with radiation 

 of approximately 13,000 and 15,000 ergs/cmVsec. show an average 

 value of 3.8, while the value for the dark controls is 7.0. In other 

 words, irradiation reduced the growth substance content of isolated 

 tips to less than half the content of those kept in darkness for the 

 same time. Ultraviolet treatment for 60 minutes on August 24 and 

 31 (table 2) also reduced the growth substance appreciably. 



A study of the growth substance content of excised Victory oat 

 tips in relation to exposure time (the intensity being constant) was 

 made on August 22. Disappearance of growth substance was pro- 

 portional to the exposure period; two hours of light reduced the 

 curvature to 3.2 as compared with dark controls of 9.5. On September 

 18 an intensity series was studied, and here again greater destruction 

 was indicated with increasing dosage of radiation (table 2). 



Zea mays. — Experiments with excised coleoptile tips of maize were 

 carried out in several dififerent ways. On September 12 and 13, 

 excised tips each 4 mm in length were placed individually in a 

 vertical position upon small blocks of 3 percent agar measuring 

 IX 1.5x1.5 mm. Some were placed in the light, others kept in 

 darkness for 120 or 240 minutes during the period of diffusion of 

 growth substance into the agar. At the end of the diffusion periods 



