128 PLANT PHYSIOLOGY 
Of the same general class of phenomena is the so-called 
hydrotropism, in which roots grow away from the dry 
and toward the moister air. 
192. In all these tropisms the stimulus must be of a 
certain strength, or it is not perceived. Even if strong 
enough to be perceived, the stimulus must act for a 
certain length of time before the plant has been suffi- 
ciently affected to bring about a reaction. The stronger 
the stimulus (up to a certain point), the shorter the time 
that is necessary for it to act. The reaction to the stimu- 
lus may be almost immediate, or it may not show itself 
for some time. In fact, the stimulus may have ceased to 
act upon the plant for some little time before the plant 
shows any response. Thus a root may be placed in a 
horizontal position for fifteen to twenty minutes and then 
restored to its normal vertical position. After a little 
while the root will begin to curve and will attain quite a 
marked curvature until the stimulus then produced by 
the resulting abnormal position induces the root to curve 
back again. In this case it usually swings too far-in the 
other direction, and does not finally attain its normal 
position until it has made several such swings. Similar 
results can be obtained with phototropism. 
193. Among the nastic movements are the opening 
and closing of flowers, in response to changes in tempera- 
ture or illumination. These are accomplished by in- 
creased growth at the base of the petals and sepals on 
the inner or outer sides respectively. A change of tem- 
perature of only one or two degrees is sufficient in the 
case of the tulip to stimulate the flower to open or close, 
as the case may be. Many plants, as long as their leaves 
are still capable of growth, show so-called sleep move- 
ments, which are not, like those of the clover (see para- 
graph 180), due to changes in turgor, but to more rapid 
‘ 
Ee —<€_~—-_— 
ii. 
