1 86 THE LIFE OF THE PLANT 



definite position : the rootlets will grow in the direction 

 of the force, i.e. away from the centre, while the stems 

 will stretch towards the centre of the wheel (fig. 52, B 2). 



Now let us see what will be the effect of a wheel rotat- 

 ing quickly in a horizontal position. Obviously the con- 

 ditions will be different here from what they were in 

 the case of the vertical rotation. There the influence 

 of the force of gravity was completely neutralised and the 

 centrifugal force alone was directive. When the wheel 

 rotates horizontally, on the other hand, both forces act. 

 The force of gravity alone would have induced the root 

 to point in the direction indicated by the arrow n 

 (fig. 52, A 2). Centrifugal force alone would have 

 caused it to point in the direction indicated by the 

 arrow m. With both forces acting simultaneously it 

 must necessarily assume an intermediate position, as is 

 shown in the figure — a position that will be nearer 

 to the horizontal the stronger the action of the centri- 

 fugal force, i.e. the larger the wheel and the more 

 quickly it rotates. Experiment fully confirms this 

 hypothesis. 



Thus the direction of the different parts of the plant 

 depends upon a force tending towards the centre of 

 the earth. By neutralising the action of that force 

 (as on a slowly rotating vertical wheel) we destroy its 

 influence. By the action of another force (as in the 

 experiment with the wheel rotating quickly in a hori- 

 zontal position) we modify the effect accordingly. But 

 only one force is known to us which corresponds to these 

 facts, and that is the force of gravity, i.e. the attraction 

 of our planet. Finally, we can cause similar phenomena 

 by substituting centrifugal force for the force of gravity 

 (as in the experiment with a wheel rotating quickly in a 

 vertical position) . We shall then see that the two organs 

 tend in the corresponding directions, i.e. the root in the 

 direction of the action of the force, the stem in the 

 opposite direction. 



