THE ROOT- TIP. 



35 



the rim of a wheel, and this, placed in a box sufficiently warm 

 and damp, was made to turn in a vertical plane at the rate of one 

 hundred and fifty revolutions a minute. After a few days, the 

 parts of the seedlings were found to be in the position shown in 



jjf^~ 



Fig. 4. Diagrams illustrating Knight's Experiments. A, wheel rotating horizontally ; 

 the plants grow under the combined influence of gravity arid centrifugal force. B, wheel 

 rotating vertically ; the direction of growth is determined by centrifugal force alone. 

 (Vines.) 



Fig. 4, b. Fig. 4, a, shows the position assumed by seedlings placed 

 under conditions entirely similar, except that the wheel was made 

 to turn horizontally. Since both gravity and centrifugal force 



