ELECTRIC RESPONSE IN PLANTS 23 
The spring-tapper.—This consists (fig. 10) of the 
spring proper (8), the attached rod (rk) carrying at its 
end the tapping-head (1). A projecting rod—the lifter 
(L)—passes through sr. It is provided with a screw- 
thread, by means of which its length, projecting down- 
wards, is regulated. This fact, as we shall see, is made 
to determine the height of the stroke. (c) is a cogwheel. 
As one of the spokes of the cogwheel is rotated past 
(L), the spring is lifted and released, and (r) delivers a 
sharp tap. The height of the lift, and therefore the 
intensity of the stroke, is measured by means of a 

Fic. 10.—THeE SpPRING-TAPPER 
graduated scale. We can increase the intensity of 
the stroke through a wide range (1) by increasing the 
projecting length of the lifter, and (2) by shortening 
the length of spring by a sliding catch. We may 
give isolated single taps or superpose a series in rapid 
succession according as the wheel is rotated slow or 
fast. The only disadvantage of the tapping method of 
stimulation is that in long-continued experiment the 
point struck is liable to be injured. The vibrational 
mode of stimulation to be presently described labours 
under no such disadvantage. 
