230 THE PLANT WORLD. 



slender pencil. Now place a healthy plant under a fine shower 

 nozzle from which water not too cold will fall in thousands of 

 repetitions upon the same leaves. The first of the mimic shower 

 causes the leaves to undergo the characteristic movements. The 

 steady, gentle tapping of the falling drops continue, however, 

 and the leaves become so accustomed or accommodated to their 

 shock that they no longer constitut-e a stimulus, with the result 

 that in a few hours the leaves are fully expanded in the falling 

 drops, the first touch of which caused a full and rapid closure 

 of all of the leaves and leaflets. The accommodating is to fall- 

 ing drops only, since if the leaves are struck with a rod, or ex- 

 posed to the action of heat from a shielded burning match, closure 

 follows. The test may be repeated by arranging a clockwork to 

 cause a small rod to strike the leaves or stems at frequent inter- 

 vals, when accommodation will follow in the same maniier. This 

 is one phase of accommodation. A second is one in which a 

 force is slowly increased. Thus, suppose that instead of sud- 

 denly exposing the plant to a shower of drops, we had placed it 

 in a damp chamber and began spraying it from an atomizer in 

 which the size of the particles of water was slowly increased un- 

 til they became large raindrops. Treatment of this kind would 

 be followed by no reaction movement, the protoplasui having 

 ample opportunity to make the necessary adjustments to the 

 size of the drops and the increased force of the blows. 



An even much more familiar illustration of stimulation is 

 that offered by the practice of storing dormant bulbs aiid tubers 

 at a low temperature, then bringing them into a warmer room 

 for sprouting. The change in the temperature is the shock which 

 awakens the resting plant in such operations, and the difference 

 between the storage pit and the growing chamber may be so 

 great that it should be made in two steps to avoid damage. On 

 the other hand the beneficial effects of such stimulation may be 

 readily appreciated when plants are stored at temperatures too 

 high to secure this shock by the change, and it also accounts in 

 part for the slow, feeble action of some species when kept at an 



