98 



BOTANY. 



rapidly (Figs. 52, 53, 54) the roots grow outward in the 

 direction of the centrifugal force, and the stems grow 

 inward, or in opposition to that force. With a slower 

 horizontal rotation (Figs. 52, 53) both roots and stems 



■w- 



Pio. 53. Fig. 54. 



Fig. 53.— Rotating apparatus driven by the hot air from a gas-jet. One 

 tenth natural size. 

 Fig. 54.— Rotating wheel driven by a jet of water. 



grow diagonally, the angle depending upon the rate of 

 revolution, but in vertical rotation the direction is not 

 changed. 



172. In considering the mode of action of gravitation 

 upon parts of plants we cannot suppose that the root-cells 

 are more subject to it than the cells of the stem. The 

 theory which affords the most satisfactory explanation 

 assumes that each cell exhibits what may be called 

 "polarity" with respect to the lines of constant force 

 (gravitation, or centrifugal force). When these lines are 

 vertical, as in the case of gravitation, the cells exhibit ver- 

 tical polarity; when the lines of force are horizontal, the 

 cells, as a consequence, arrange themselves horizontally; and 



