260 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 30 



For this purpose a wooden light-proof box, 7 feet 10 inches long 

 and 12 inches wide, was built. This box was divided into five com- 

 partments or separate chambers. The two end compartments con- 

 taining the light sources were approximately 27 inches high, or 5 

 inches higher than the other three. The central, or plant compart- 

 ment, was 2 feet 10 inches long. Between the central and the end 

 chambers were partitioned off two small (*bmpartments containing 

 ray filters and the water and air cooling devices. This apparatus, 

 the plant photometer, is illustrated in Plate 2. 



Briefly, the method of carrying out this experiment is to place, for 

 example, a yellow ray filter in one end of the central or plant cham- 

 ber and a green one in the other end. The intensities of the lights 

 coming from the lamps in the end chambers were so adjusted that they 

 register the same intensity on a delicate thermocouple placed at the 

 center of the plant compartment. After the proper adjustment had 

 been made the light measuring apparatus was removed, as illustrated 

 in the picture, and the seedling of an oat plant placed in the chamber. 

 The seedling was placed in a vertical position and surrounded by a 

 rotating double-walled glass cylinder. This cylinder, rotating slowly 

 around the seedling, insures equal temperature and moisture condi- 

 tions in the immediate vicinity of the plant. This precaution is 

 necessary since slight differences of temperature or moisture will 

 cause unequal growth in the side of the stem and bring about a 

 curvature. 



After the seedling has been properly adjusted between the yellow 

 and green lights of equal intensity the box is closed and the plant 

 permitted to grow from two to four hours. When the plant was 

 examined in this particular experiment, it had grown toward the 

 green light somewhat in the manner of the seedling illustrated in 

 Plate 3. Thus, of the two colors of equal intensity used in this 

 experiment the green was more effective in retarding growth, as 

 was evident from the plant's bending toward that light. 



In order to evaluate the effect of one color in terms of another, 

 other experiments were carried out after changing the relative in- 

 tensities of the two lights. Intensities were finally reached at which 

 the plant grew without bending toward either light. The intensities 

 were then measured, and it was found that the yellow light was 

 approximately 1,000 times as intense as the green at the balancing 

 point. The light passing through the blue ray-filter used in these 

 preliminary experiments exerted a greater growth-inhibiting influ- 

 ence than either the yellow or green lights, the factor being 30 when 

 compared with green or 30,000 when compared with yellow, assuming 

 a linear intensity effect. We see, therefore, that the effect becomes 

 much more jDronounced as we shorten the wave length. 



In order to more completely describe this increase of phototropic 

 effect with decrease in wave length, that is, in passing from red to 



