136 



RELATIONS OF PLANTS TO LIGHT 



ond on the opposite side at a distance of two meters. If a reac- 

 tion to the second is secured, repeat the test moving the second 

 candle farther away until the response disappears. Note the 

 difference in the distances of the two candles, and calculate the 

 percentage of difference necessary to constitute a stimulus. If no 



reaction is secured to the 

 second candle at two meters, 

 it must be moved closer. 

 Fresh lots of seedlings 

 should be used in every test, 

 or several hours allowed to- 

 elapse between tests if the 

 same are used. 



207. Zone of Curvature, 

 Secure a number of young 

 plants of Helianthns, Zea, 

 Avena, Lepidium and others 

 and mark the tips of the 

 stems into intervals of 5 

 mm. by means of India ink 

 applied with a thread. Grow 

 the specimens in the dark 

 room for a day to relieve 

 the apical regions of all pho- 

 totropic curvatures. Mea- 

 sure the distances between 

 the intervals on the stems, 

 and set the plants in a 

 position before a window 

 through which they will re- 

 ceive a strong illumination. 

 Note the region of curvature 

 a few hours later and find 

 what relation it bears to the 

 zone of greatest growth. 



FlG. 68. Seedling of Sinapis alba in dish 

 of water exposed to light from one direction 

 only, indicated by arrows, c, c, diaphoto- 

 tropic cotyledons, from which the impulse is 

 transmitted to the motor zone of the hypocotyl 

 at a. b, base of hypocotyl. n, n, level of 

 water, d, motor zone of root, showing apho- 

 totropic curvature. This curvature was pro- 

 duced some time earlier, and the perceptive 

 zone of the root "has been carried some dis- 

 tance away by the growth of the tip. After 

 Sachs. 



