PHOTOTROPISM — JOHNSTON 315 



tive. Furthermore, it has been shown in recent years that there 

 are present in the plant specific growth substances which are directly 

 associated with this light-growth response. 



The foundation of much of the recent work on phototropism was 

 laid by the Dutch investigator, Blaauw. Perhaps the first quantita- 

 tive measurements and physical interpretations were made by him 

 and published in 1909. The responses of young seedlings were stud- 

 ied in different regions of the spectrum, and the energy values were 

 calculated from Langley's tables. For oat seedlings Blaauw found 

 the most effective region of the carbon arc spectrum to lie between 

 4,660 and 4,780 A, whereas the red and yellow regions were ineffec- 

 tive. The minimum amount of radiation required to produce photo- 

 tropism was 20 meter-candle seconds. Furthermore it appears that 

 for equal effects the product of light intensity and duration of ex- 

 posure is a constant. 



Dillewijn, another Dutch investigator, has shown in a very inter- 

 esting manner that the phototropic response of a plant can be pre- 

 dicted by knowing how it grows in light of different intensities. He 

 thus shows that phototropism is a light-growth response, as brought 

 out by Blaauw's theory. Using oat seedlings, Dillewijn determined 

 the rate of growth in several different " quantities " of light and also 

 for those of one-thirtieth the value. He had calculated that the light 

 falling on one side of a seedling was reduced to one-thirtieth of its 

 intensity after passing through to the opposite side. This value, of 

 course, varies with the plant used. 



The results of Dillewijn's experiments are reproduced in figure 1. 

 The abscissa represents time in hours, and the ordinate the rate of 

 growth expressed as fi per minute (/*= 0.001 mm). The light intensity 

 is indicated on the right in meter-candles. The arrow represents the 

 time of illumination, with the duration noted above in seconds. 



Each pair of curves represents two light " quantities ", one one- 

 thirtieth of the value of the other shown as the lightly and heavily 

 drawn lines, respectively. In the d pair of curves the side of the 

 seedling next to the source of illumination (heavy line) is retarded 

 more than the far side (light line), which results in a positive 

 bending for this weak light quantity (80 and 2.5 meter-candles for 

 10 seconds). With a greater quantity of light (2,400 and 80 meter- 

 candles for 10 seconds), as illustrated by the pair of curves in c, 

 first a positive then a negative bending would occur. A further 

 increase (2,400 and 80 meter-candles for 90 seconds) will again bring 

 about a positive bending as in h which becomes less as the quantity 

 is increased, as shown graphically in a (2,400 and 80 meter-candles 

 for 15 minutes). These predicted growth curvatures correspond to 

 real curvatures that have been found, and they indicate that with 



