132 RELATIONS OF PLANTS TO LIGHT 



phate in 100 cc. water and use in layer 20 mm. thick. The 

 potassium permanganate should be freshly made when used, and 

 the color solutions should be kept in the dark or in opaque glass 

 bottles. 



201. Reaction Time. Bring a rapidly growing plant from the 

 dark room in which it has been placed for a day, and set it near 

 a window from which it will receive a strong light. Set a hori- 

 zontal microscope with its barrel parallel to the window and 

 quickly focus on some part of the tip of the terminal bud. Note 

 the length of time elapsing before movement of shoot toward 

 source of light takes place. Seedlings a few centimeters in 

 height will be most suitable for this test. 



202. Critical Points in the Phototropic Relations of Light to 

 Plants. The intensity of light necessary to constitute a stimulus 

 varies enormously in different species according to the degree of 

 sensitiveness which they have acquired and the stage of develop- 

 ment. A " normal " candle burning 7.78 grams of paraffine per 

 hour and standing one meter from the sensory zone may be 

 taken as a standard. The intensity of illumination decreases as 

 the square of the distance from the flame. Lepidium sativum has 

 been found to respond to .00033 meter candle illumination in a 

 dark room, and the minimum varies in different species to .06 

 meter candle in Raphanus sativus and others. The intensity 

 necessary to secure the fullest reaction constituting the optimum 

 varies from . 1 1 meter candle in Pisum and Phaseolus epicotyls to 

 .44 meter candle in the epicotyl of Vicia. The optimum is gen- 

 erally much higher in etiolated plants, which are also less sensi- 

 tive to geotropic stimuli. An increase of the intensity of illu- 

 mination a hundred or even a thousand times is necessary to reach 

 the maximum, or point beyond which the reaction ceases. 1 



Increase of the intensity of illumination above the maximum 



1 Figdor, W. Versuche ueber die heliotropische Empfindlichkeit der Pflanzen. 

 Sitzungsber. Akad. d. Wiss. Wien. 102 : 45. 1893. 



Wiesner, J. Photometrischen Untersuchungen auf Pflanzen physiologischen Ge- 

 biete. Sitzungsber. Akad. d. Wiss. Wien. 102 : 291, 350. 1893. 



