96 EXPERIMENTAL PLANT PHYSIOLOGY 



that would otherwise reach the roots, the box need not 

 be tilted at so great an angle. 



"" primary root grows straight 

 The secondary roots 



Observations. — The 

 down to the base of tlie box 



Fig. 47 



grow out almost at right angles to the primary root, 

 while the shoot grows Tertically upwards (Fig. 47). 



By the foregoing experiments it has been proved 

 that the main root of a plant always turns downwards 

 when allowed to grow naturally in damp soil. The 

 cause of this downward growth must now be investi- 

 gated. 



When a body is dropped it falls at once to the 

 ground. This is a direct result of a force by which 

 everything is attracted to the centre of the earth. 

 This force is termed the Force of Gravity, and the 



