IRRITABILITY. 55 



when their tips are pointing upward at an angle of 45 degrees 

 from the vertical ; apogeotropic organs respond most readily 

 when pointing downward at an angle of 45 degrees ; and dia- 

 geotropic organs respond with equal facility in either position. 

 The action of gravity upon a plant may be neutralized by plac- 

 ing it on the periphery of a wheel revolving in a vertical plane, 

 or by turning the plant on its own axis in a horizontal position. 



EXPERIMENT 67. 



NEUTRALIZATION OF INFLUENCE OF GRAVITY. 



Take the hands from a large wall-clock whose dial is parallel to 

 the rays of sunlight (to avoid the disturbing action of light) and 

 fasten to the prolonged axis a cork plate 10 cm. in diameter, or a 

 wheel to the periphery of which are attached pieces of cork. 

 Fasten seedlings of Pea to the cork in various positions by means 

 of pins, and set the clock in motion. Twenty-four hours later each 

 seedling will be found to be growing in the position in which it was 

 placed, and no marked curvatures in any of the organs can be 

 noticed. (Fig. 50.) 



Bemark. — The revolving wheel must be partially immersed in water or 

 placed in a spray to keep the seedlings moist. A small American clock 

 can be used instead of the large clock shown in the figure. 



45. Replacement of Gravity.— When gravity is overcome by 

 another force, the seedling tends to place its axis parallel to this 

 new force in the same manner as toward gravity in its normal 

 position. The force which acts upon a plant fastened to a 

 rapidly-revolving wheel acts in a tangential direction ; conse- 

 quently the plant tends to place its axis parallel to the tangent. 

 This is true of plants rotated in a vertical plane at a speed of 

 100 to 300 revolutions per minute with a wheel having a radius 

 of 6 to 20 cm. If a plant is rotated in a horizontal plane at 

 this speed, centrifugal force tends to cause the shoot axis to 

 lie in a horizontal plane, while gravity tends to cause the 

 axis to take a vertical position. In this instance the axis will 

 take a position between the direction of these two forces. 



