NO. 20 GROWTH SUBSTANCE BURKHOLDER AND JOHNSTON 3 



were exposed to moderately bright skylight for 5 hours (to keep the 

 first internode short during subsequent growth) and then transferred 

 into the darkroom for the rest of their development. Some 48 hours 

 after the first soaking in water, the seedlings were placed in glass 

 holders and the coleoptiles allowed to grow vertically straight in the 

 humid darkroom for 30 hours. Then the seedlings were selected for 

 size and straightness and arranged in uniform racks. In 75 to 80 

 hours from the beginning of germination the coleoptiles attained 

 about two-thirds their final size and were ready for use. 



Maize seeds were soaked in water for 4 hours and then put to 

 germinate in moist soil in the darkroom. About 100 to 120 hours 

 were required to obtain maize coleoptiles two-thirds their final size, 

 and suitable for experimentation. 



The Avena test plants were decapitated 40 minutes before they were 

 to be used in the biological assay of growth substance. Small pieces 

 of plant tissue or agar blocks containing unknown amounts of the 

 substance were tested by applying them unilaterally to the cut surfaces 

 of the coleoptile stumps. Growth curvatures at the end of 2 hours 

 were recorded as shadow pictures on photographic paper (pi. 2, 

 lower), and later the angles were measured with a protractor. The 

 average Avena curvature in degrees is recorded in the tables of data. 



The sources of radiation employed were a mercury in quartz arc 

 operating on 220 volts D. C, and a Mazda 60-watt tungsten lamp 

 run on no volts A. C. White light from the arc, designated as 

 " total arc " in the tables, was filtered through 5 cm of distilled 

 water contained in a glass cell in order to remove short ultraviolet 

 and long infrared wave lengths. The blue mercury line, 4358 A, was 

 isolated with Corning glass line filter no. 7, 3.06 mm and 3.25 mm thick, 

 in addition to the water filter. Ultraviolet radiation from the mercury 

 arc was passed through only a no. 986 Corning red-purple Corex " A " 

 filter 7 mm in thickness. Excised coleoptile tips and agar blocks 

 during exposure were kept in moist glass chambers lined with wet 

 Ijlotting paper. When ultraviolet rays were used, thin quartz covers 

 were placed over the moist chambers to permit entrance of short 

 wave lengths. With the Mazda lamp, no filters were used except the 

 3-mm-thick glass wall of the moist chamber, which removed some 

 infrared rays. 



In each case the energy incident upon the plants was measured 

 with a thermocouple and galvanometer calibrated with a standard 

 lamp. Radiation from the arc was allowed to fall vertically through 

 the appropriate filters directly upon the plant materials shielded from 



