4] RHEOTROPISM 387 



rapidly in water than normal ones. In like manner, if the 

 root is, as it were, half decapitated, by cutting the tissue on 

 one side, an abnormally rapid growth will take place on that 

 side, proximal to the injury. The reason for this more rapid 

 growth, continues WIESNER, is that the nutritive fluids 

 intended for the tip are prevented by the injury from attaining 

 the tip, and, consequently, go to build up the cell-walls above 

 the wound. While the possibility of this explanation cannot 

 be denied, the facts are not opposed to another interpretation, 

 such as is offered by SPAULDING, and which is more in accord 

 with the explanations of other tropic phenomena. This expla- 

 nation is that the wounding produces a chemical change in the 

 protoplasm of one side of the root, such that growth occurs 

 more rapidly upon that side, either as a result of increased 

 upbuilding of cell-walls or of increased imbibition of water, or 

 of both. 



4. EFFECT OF FLOWING WATER UPON THE DIRECTION 

 OF GROWTH - - RHEOTROPISM 



When the radicle of a bean is suspended by the cotyledons 

 above a stream of flowing water, so that it lies in the axis of 

 the current and points down stream, the free end, as it grows, 

 gradually turns, and becomes directed up stream. It turns 

 against the current. 



This remarkable phenomenon, named rheotropism by its dis- 

 coverer, JONSSOX ('83, p. 518), has been carefully studied by 

 him bv the following method : The stream, whose rate could 



v d7 



be controlled at will, flowed in a trough. Over the current, 

 seedlings of maize and other grains, with well-developed rad- 

 icles, were suspended so that the radicles lay in the axis of the 

 current, and were directed either up stream or down as desired. 

 When the rootlets were directed down stream, a turning began, 

 after a latent period of several hours, and reached its final posi- 

 tion in the current in about twenty hours. If the rootlet was 

 originally directed up stream, it simply grew straight ahead 

 until mechanically bent out of position by the impact of the 

 water. By directing a rootlet alternately down stream and up 

 stream after each rheotropism has occurred, the whole root 

 may become very zigzag. These facts show clearly that cer- 



