158 THE GROWTH OF ROOTS AND STEMS. 



tendencies of root and stem are to be ascribed to gravity, we 

 should expect similar tendencies to be exhibited under the 

 action of centrifugal force. This is the case; in seedlings 

 fixed to the vertically revolving wheel, the roots bend away 

 from the centre of the wheel and grow straight out in the 

 direction of the radius, while the stems grow straight in 

 towards the centre of the wheel. 



What will happen if the wheel revolves horizontally ? Two 

 forces now act on the plant, the centrifugal force and that 

 of gravity, and the root bends obliquely outwards, the stem 

 obliquely inwards. 



Side-roots are but little sensitive to gravity ; they grow out 

 from the parent root, each growing in a definite direction. 

 If the growth of a side-root is towards, say, the east, and it is 

 interrupted by a stone, as soon as the root has reached the 

 edge of the stone it will resume its eastward direction of 

 growth. The branches of the side-roots, again, grow away 

 from these, and may therefore grow in a downward, or 

 horizontal, or even upward direction. 



We have also found by experiments that the attraction- 

 stimulus of gravity may, even in the case of a main root, be 

 overcome by the "attraction "-stimulus of moisture or by the 

 "repulsion "-stimulus of light, while contact with an obstacle 

 like a stone causes the root to change its direction of growth 

 for a time, though it resumes this direction as soon as it 

 reaches the edge of the obstacle. It is clear, therefore, that 

 the root is a sensitive organ, its sensitiveness or " irritability " 

 being much the same as that of an animal. 



It is very important to understand that geotropism and 

 other forms of irritability in plants answer to the various 

 kinds of sensation in animals, and also to try to see the reason 

 for each response made by each part of a plant to each kind 

 of stimulus from outside it. If you have carried out the 

 preceding experiments, stop here and think out for yourself 

 why i.e. to what advantage the main root of a bean or pea 

 (or other plant you have worked with) grows downwards, 

 why it grows towards moisture, why it grows away from light, 

 why a root turns off on meeting an obstacle and goes on its 

 former course after getting to the obstacle's edge, why a 

 branch-root grows away from its parent-root. In each case 

 remember the two great functions of the root (what are 



