10 LIVING PLANTS 



tendency of growing organs to extend in a 



straight line unless acted upon by outside force. 



There is only one force known that acts uni- 



ovcry o formly in the direction of the center of the 

 gravity sense , ^ • 



earth, that is gravity ; and it was the genius 



of Andrew Knight, an Englishman, to demon- 

 strate as long ago as 1^06, that this force 

 does furnish the directive influence in securing 

 verticality to plants. He grew plants on re- 

 volving wheels, and found that they respond- 

 ed to centrifugal force, and that when the 

 wheel was placed horizontally and revolved 

 at a speed that made the centrifugal force 

 equal to that of gravity, both roots and 

 stems grew obliquely, taking the position of 

 a resultant of the two forces, that is, of forty- 

 five degrees to the vertical. 



But this discovery by Knight was not imme- 

 diately fruitful, for no one could tell how grav- 

 ity produced the effect ascribed to it. If it 

 pulled the root down, why did it push the 

 stem up? The stem is as heavy as the root, 

 why are not both attracted toward the center 

 of the earth? It was a curious paradox to 

 say the same force acted now one way and 

 now exactly the reverse on different parts of 

 the same plant ; as if pulling and pushing 

 were the same thing. It was supposed that 

 gravity acted upon the root as it does upon a 

 mass of taffy candy, drawling it downward. 



