2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 92 



direction theory advanced by Sachs in 1876 and supported by the 

 experiments of Strasburger, Davenport, and Canon. 3. The wave- 

 length theory first investigated by Payer in 1842. 4. The energy 

 theory first mentioned by Miiller in 1872 in which the maximum 

 response of cress seedlings shifted in the spectrum for different 

 energy values of the wave lengths studied. 



The basis for much of the recent quantitative work on phototropism 

 was laid by Blaauw (1909, 1914, 191 5, 1919)- His studies were 

 perhaps the first serious attempt made to interpret this growth re- 

 sponse in terms of modern physics. Plant responses were studied in 

 different spectral regions of sunlight and of the carbon arc and com- 

 pared with the energy values calculated from Langley's (1884) tables. 

 Blaauw found the most effective region of the carbon spectrum for 

 phototropic response of Az'ena seedlings to lie between 4660 and 

 4780 A, while the red and yellow regions were ineffective. According 

 to Blaauw (1914), the curvature of a plant resulting from unilateral 

 illumination is caused by the light-growth responses on the opposite 

 sides which are illuminated differently. The minimum amount of 

 radiation required to produce phototropic response was found to be 

 20 meter-candle-seconds. It also appears from his work that for 

 equal effects the product of light intensity and time of exposure is a 

 constant. 



It is impossible to evaluate the effect of wave length in manj^ of the 

 early phototropic experiments because of the lack of accurate physical 

 data. Some 10 years after the early quantitative studies of Blaauw. 

 Parr (1918) made a study of the responses of Pilobohis to different 

 wave lengths and intensities of carefully measured artificial light. 

 The results of these quantitative studies are best summarized in her 

 own words : 



(i) Pilobohis responds to the light of all the regions of the visible spectrum. 

 (2) The presentation time 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 vi^ith 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 con- 

 stant 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. 



