THE GROWTH OF ROOTS AND STEMS. 157 



189. Test-Tube Experiments. For demonstration to a 

 class, some of the foregoing experiments may be duplicated by 

 arranging them on a small scale in test-tubes, which are handed 

 round the class. The following will serve as examples : 



(a) Get a test-tube with a cork to fit, pin to the cork a Pea-seedling 

 with a straight root 2 ins. long, put a strip of blotting-paper in the 

 tube, and run in water to soak it. Fix the seedling in the tube with 

 its root pointing to the closed end, and keep the tube in a vertical 

 position, with the corked end uppermost. 



(6) Fix a similar seedling in a tube, but keep the tube inverted, so 

 that the root points vertically upwards. 



(c) Get a similar seedling and tube, but with a razor or sharp knife 

 cut off the extreme tip of the root, and keep the tube in the same 

 position as in (6). 



19O. The Root-Tip a Sensitive Organ. From the 

 foregoing experiments it is clear that the root responds, by 

 changing its direction of growth, to the influences or stimuli 

 of moisture, light, and contact. We have also seen that the 

 main roots always grow vertically downwards, towards the 

 centre of the earth, except when deflected from this direction 

 by one of the stimuli just mentioned. It will be noticed that 

 the shoot of a seedling grows towards light and away from 

 moisture, its responses to these stimuli being the opposite of 

 those made by the main root. The force of gravity also has 

 a stimulating effect on the growth of root and shoot ; the root 

 responds by growing down in the direction of the force, the 

 stem by growing up in the opposite direction. 



That these opposite tendencies of root and stem are to 

 be ascribed to gravity has been determined by experiment. 

 There is a machine called the clinostat, consisting essentially 

 of a plate mounted on a horizontal axis, and therefore rotating 

 vertically. A plant is attached horizontally to the plate, and 

 the axis is rotated slowly a revolution in from 15 to 30 

 minutes. A little reflection will show that the influence of 

 gravity is eliminated. Stem and root simply continue to 

 grow straight onwards in whatever direction they have been 

 placed at the beginning of the experiment. 



Another experiment is to attach a plant to the edge of a 

 wheel which rotates rapidly and vertically. Here another 

 force centrifugal force comes into play. If the opposite 



