1080 



BIOLOGICAL EFFECTS OF RADIATION 



in conjunction with a copper chloride filter. The most sensitive region 

 proved to be in the violet (4000 to 4300 A). In Fig. 4 the author 

 compares his results with those obtained by Blaauw and Parr. It is 

 pointed out that because of the presence of "accessory" pigments in 

 these sporangiophores care must be taken in correlating these results 

 with those obtained from the absorption spectrum of the photosensitive 

 substance. 



The early results of Johnston, Brackett, and Hoover (24) justified 

 a more elaborate and accurately controlled experiment wherein narrower 



spectral regions could be used. 

 For this purpose Johnston 

 (23) used a specially con- 

 structed monochromator and 

 exercised considerable care to 

 eliminate scattered light and 

 to keep the conditions sur- 

 rounding the coleoptile sym- 

 metrical, with the exception 

 of the wave-length region 

 being investigated. A 

 double-walled glass cylinder 

 with water between the walls 

 slowly rotated about the axis 



The entire 



I.IO 

 1.00 

 0.90 

 0.80 

 0.10 

 0.60 

 0.50 

 Q40 

 030 

 0.20 

 O.IO 



1 1 1 1 1 1 1 1 1 \ I ^Asm^m^ I CW 



4.000 4.200 4,400 4.600 4300 5.000 5,200 5.400 

 Fig. 5.— Graph showing phototropic sensitivity 



curve. The ordinates are relative sensitivity values, /. i 



the abscissas, wave-lengths in Angstroms, and the Ol the COleoptllC. 



horizontal bars indicate the wave-length ranges of cylinder WaS enclosed in a 



the balance points. Circles indicate points obtained ,. , r i u- u 



with filters combined with the monochromator. llght-proof boX whlch COn- 



The crosses show where mercury lines were used, tained twO oppositely placed 



{From Johnston, 23.) . , . , rrn i 



Side wmdows. 1 hrough one, 

 light from the monochromator was passed, and through the other, 

 light from the standard lamp. The standard light used was a 200- 

 watt, 50-volt projection Mazda lamp with the filaments in a plane. 

 This light was passed through a Corning line filter (No. 6.0), a heat- 

 absorbing glass, and a water cell. Its radiation intensity was 0.37 

 /iw/cm.^ at a distance of 25 cm. The general method of procedure 

 was similar to that used with the earlier type of plant photometer. 



The data from this more accurately controlled experiment are pre- 

 sented in the table shown on page 1081, and shown graphically in 

 Fig. 5. 



The phototropic sensitivity curve rises sharply from 4100 A to a 

 maximum of 4400 A. It then drops off to a minimum at about 4575 A 

 and again rises to a secondary maximum in the region 4700 to 4800 A. 

 The fall is very rapid from this point to 5000 A, thence it tapers off 

 very gradually to a threshold on the long-wave-length side at about 

 5461 



