IRRITABILITY. 



417 



ness may be largely due to the action of light, gravity, etc. 

 In order to determine whether or not there is any inherent 

 tendency in a plant-organ to grow straight, it must be grown 

 under such conditions that neither light, nor gravitation, nor 

 any other external agent, can exercise any directive influence 

 upon it 



The directive influence of an external agent can be eliminated in 

 either of two ways ; by removing the agent altogether, or by arranging 

 that all sides of the plant- organ shall be equally exposed to its influence. 

 Thus, the action of light can be got rid of by placing the plant in dark- 

 ness, or by causing it to revolve so that each side of the organ is exposed 

 for equal periods of time to light falling upon it in any given direction. 

 Similarly the effect of the action of gravity is eliminated by causing the 

 organ to rotate slowly round a horizontal axis, so that each side of the 

 organ makes any given angle with the line of action of gravity for equal 

 periods of time. 



The instrument for slow rotation of this kind is known as the " Clino- 

 stat," and we owe its introduction into experimental physiology to Sachs. 

 Annexed is a figure of a convenient form of clinostat. 



Er 



FIG. 48 (after F. Darwin). The Clinostat. 



It consists of a stand, 6, bearing two upright supports, s, s, on which 

 the spindle, k, rests. The spindle bears near one end the two driving- 

 wheels, W and w, over either of which the driving-cord, dr, connected 

 with the clock-work below, may be led. The spindle rests towards its 

 other end on friction-wheels, /r, and is screwed on to a metal plate which 

 plays freely between the metal plate m and the disc of wood, //, to which 

 the box /? (containing in this case a flower-pot) may be fixed by means of 



v. 2; 



