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 



