GROWTH MOVEMENTS 1077 



lamp and a 200-\vatt nitrogen-filled tungsten Mazda lamp. The results 

 of these quantitative studies are best summarized in her own words: 



(1) Pilobolus responds to the light of all the regions of the visible spectrum. 

 (2) The presentation thne decreases gradually from red to violet. There is no 

 indication of intermediate maxima or minima. (3) The presentation time does 

 not vary in direct ratio with the measured value of the energy of the light in the 

 different regions of the spectrum. (4) The presentation time varies in inverse 

 ratio to the square roots of the wave frequency. (5) The product of the square 

 root of the frequency times the presentation time, decreases with the decrease 

 in the energy value of the spectral regions, and is an approximate constant for a 

 given light source. (6) The spectral energy in its relation to the presentation 

 time may be expressed approximately in the Weber-Fechner formula, if the wave 

 frequencies be made a function of the constant. (7) The relation of the spectral 

 energy to the presentation time may also be approximately expressed in the 

 Trondle formula, the wave frequencies being made a function of the constant. 



Hurd (21) equalized the intensity of light coming through a series of 

 Wratten light filters so that it measured 1800 meter-candles on reaching 



o 



the young rhizoids used in this work. Only the blue (4700 to 5200 A) 

 and violet (4000 to 4700 A) lights produced phototropism, negative in 

 direction. The other lights at this intensity had no effect. With a 

 greater intensity the green light (5200 to 5600 A) exerted a negative 

 phototropic effect as well as the blue and violet. 



For the purpose of investigating the wave-length effects of radiation 

 on phototropic bending of young plants Johnston (22) described a simple 

 plant photometer which w^as later improved and used (24) in evaluating 

 four spectral regions. The general procedure was to place a seedling 

 between two different and oppositely placed lights and, after an interval, 

 observe the growth curvature. If, for example, the seedling was exposed 

 to blue and 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 

 imtil 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 

 intensities w^ere measured. From these experiments with oat seedlings 

 no measurable phototropic response was found for wave-lengths longer 



o 



than 6000 A while a noticeable bending was found with the yellow filter. 

 The threshold for wave-length influence seemed to originate somewhere 

 between 5200 A and 6000 A. The effects of green and blue were pro- 

 gressively greater, being respectively in round numbers 1000 and 30,000 

 times that of yellow. 



Sonne (40) determined the amount of energy of different wave-lengths 

 necessary to bring about a minimum phototropic action in oats. About 

 1 cm. of the tips of young plants were placed at different distances from 



