6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



sporangiophores were placed between two light sources. The intensi- 

 ties were adjusted until the phototropic effects of the different 

 spectral regions were equal. At this point the efficiency of each region 

 was taken as proportional to its relative energy content. Wratten 

 fiiters were used in conjunction with a copper chloride filter. The most 

 sensitive region proved to be in the violet (4000-4300 A). In figure 2 

 Castle compares his results with those obtained by Blaauw and Parr. 

 It is pointed out that because of the presence of " accessory " pig- 

 ments in these sporangiophores care must be taken in correlating these 

 results with those obtained from the absorption spectrum of the 

 photosensitive substance. 



PRELIMINARY EXPERIMENTS 



The general method of studying the wave-length effects on photo- 

 tropism as described by Johnston (1926) was used by Johnston, 

 Brackett, and Hoover (1931) with an improved plant photometer for 

 evaluating four spectral regions in terms of plant response. The gen- 

 eral procedure was to place an oat seedling between two different and 

 oppositely placed lights, and after an interval observe the growth 

 curvature. If, for example, when the seedling was exposed to blue and 

 to green lights, a distinct bending was noted toward the blue side, 

 the lights were so adjusted as to increase the green or decrease 

 the blue intensity. Another seedling was then used and the process 

 repeated until a balance point was reached where the effect of one 

 light neutralized the effect of the other. When this balance point was 

 determined, a specially constructed thermocouple replaced the plant 

 and the relative light intensities were measured. From these experi- 

 ments it was found that no measurable phototropic response was 

 found for wave lengths longer than 6000 A ( Wratten no. 24 — red 

 filter), while a noticeable bending was found with the yellow filter 

 (Coming's heat-resisting yellow — yellow shade), whose cut-off on 

 the short-wave-length side was 5200 A. The threshold for wave- 

 length influence was found to lie somewhere between 5200 and 

 6000 A. The effects of green and blue light (Wratten filters nos. 61 

 and 47 respectively) were progressively greater, being in round num- 

 bers 1,000 for the green and 30,000 for the blue times that of the 

 yellow. 



These results justified a more elaborate and better controlled ex- 

 periment wherein narrower spectral regions could be investigated. 

 For this purpose Johnston (1931) used the specially constructed 

 monochromator illustrated in plate i. Care was taken to eliminate 

 scattered light and to keep the conditions surrounding the coleoptile 



