THRESHOLD OF STIMULATION 135 



note the distribution of the organism through the culture. Now 

 move the microscope to a distance of three meters from the win- 

 dow and look for changes in distribution. A half hour later 

 move to within 1.5 meters of the window and observe. Next 

 move directly up to window, but not in sunlight. If suitable 

 conditions are offered the maximum may be found, and as the 

 preparation is brought nearer the window the organisms move 

 away from the source of light, thus exhibiting aphototaxis, while 

 at lower intensities they were prophototactic. Interesting tests 

 may be made with zoospores of all kinds. 1 



205. Summation of Stimuli. Grow seedlings of any conve- 

 nient species in the dark room until they have attained a height 

 of 24 cm. and then expose to illumination of a small candle or 

 burner, to ascertain the minimum amount of time necessary to 

 secure a reaction. Now secure a second lot of seedlings, and ex- 

 pose them to an illumination of the same flame for a period of 

 one-fifth of the presentation time determined, then shade the 

 flame for a period equal to one-tenth of the presentation time, 

 repeating the alternation of periods of illumination and darkness 

 a half dozen times in an effort to ascertain what repetition of an 

 amount of light, not sufficient to produce a response, may secure 

 a response by a summation of effects. 



The experiment may be made in another form if an electric spark- 

 ing apparatus is at hand. The seedlings may be subjected to the 

 illumination of a definite number of sparks at regular intervals. 



206. Threshold of Stimulation. When a plant is subjected to 

 light from one source striking it on one side, a certain increase 

 over this intensity will be necessary in a light coming from the 

 opposite direction in order to set up a new reaction. Place a 

 candle or microburner at a distance of three meters from a seed- 

 ling in a dark room until a curvature is produced, then set a sec- 



1 Holt, E. B., and Lee, F. S. The theory of phototactic response. Amer. Jour. 

 Physiol. 4: 460-481. 1901. 



Oltmanns, F. Ueber positiven und negativen Heliotropismus. Flora, 83 : i. 

 1897. 



