154 THE GROWTH OP ROOTS AND STEMS. 



185. What makes the Main Root grow downwards 

 and the Main Stem upwards ? Most likely this question 

 would not have occurred to you at all, or if it had you would 

 probably have thought of anything but the right answer or 

 the right way to test the matter. It is clear that the root 

 does not simply bend down by its own weight, like a rod of 

 plastic material, for it exerts great force in its downward 

 growth. Besides, this "explanation" would not apply to the 

 shoot, nor even to the side-roots ; the former grows upwards, 

 the latter grow away from the parent-root. 



If the young root is placed horizontally, it soon begins to 

 bend downwards, the curvature taking place just behind the 

 growing apex. This curvature is soon rendered permanent 

 by the growth and hardening of the tissues affected, but a 

 new terminal straight portion of the root is soon formed as 

 growth continues, and if this be again laid horizontally a 

 downward curvature is produced as before. This curvature 

 is due to the fact that gravity, the attractive force which 

 the earth exerts on all bodies, acts as a stimulus upon the 

 protoplasm at the apex of the root, and causes the growing 

 region to bend towards the centre of the earth. 



The growth- curvatures by which root and shoot respond to 

 the stimuli of gravity, light, moisture, etc., are called tropic 

 movements (Greek tropos, direction). In most plants, the 

 root is positively geotropic, negatively heliotropic, and posi- 

 tively hydrotropic, while the shoot is negatively geotropic 

 (response to gravity), positively heliotropic (response to light), 

 and negatively hydrotropic (response to moisture). 



* (a) That it is not the action of the soil which causes the root to grow 

 downwards is readily shown, e.g. by growing seedlings in moist air. 

 Choose a few Bean seedlings with straight roots about 1^ ins. long. 

 Dry the roots gently by stroking with pieces of blotting-paper, and 

 mark each radicle at intervals of ^ in. by making transverse lines with 

 Indian ink, beginning at the tip. Pin the seedlings to a piece of wood 

 or a cork, and invert it over a wide-mouthed jar containing a little 

 water. Fix different seedlings with the radicle pointing upwards, 

 downwards, and horizontally. From day to day observe and sketch 

 the appearance of the seedlings, noting the direction of growth of the 

 shoot and the root. 



(b) Grow Bean or Pea seedlings in your box with a sloping glass 

 front. When the side-roots have grown out, mark on the glass the 

 positions of a few of these, also of the main root ; note especially the 

 position of the tip of each root. Then tilt the box up at an angle of 



