20 Plants and their Ways in South Africa 



seen that the root tends to grow towards the centre of the 

 earth. The side roots extending obliquely in several direc- 

 tions, are well placed for obtaining their food. Pinch off a tip 



of the main root. One of 



the side roots now bends 



down to take its place. 



Which root does this? 



Is more than one root 



affected? 



How does the root 



curve ? 



Ex. 5. Mark the roots of 

 seedlings in the germinator as 

 before. When the roots are an 

 inch or two in length, suspend 

 the germinator from the side. 

 Now observe where the bend 

 occurs. How does the shortest 

 curve compare with the position 

 of greatest growth ? 



Mark other roots, and with 



FIG. 28.-Stem curvature induced by a shar P knife cut off the extreme 

 gravity. t'P s OI some. 1 Early the next 



day notice the difference be- 

 tween the cut roots and the uninjured ones. None of the cut roots have 

 bent downward. They have lengthened, so that we know the growing 

 region or motor zone has not been injured. The root grows down 

 because it is stimulated by gravity. But from our experiment we have 

 seen that the tip is the part which is sensitive to this stimulus. It is called 

 the perceptive zone. 



Ex. 6. The Direction of the Stem. The stem has as strong an up- 

 ward growth as the root has a downward tendency. Place seedlings that 

 have grown in pots or in the germinator in a horizontal position. In 

 three or four hours a decided change of direction has taken place. In 

 Fig. 28 notice that the one at a is curving, although the end was cut. 

 Observe the stem curvatures of monocotyledons. Where does curvature 

 take place in the jointed stems of grasses ? In Commelina ? 



After the seedlings have become upright, turn the pot half-way around. 

 Note how soon a change of direction may be observed. It will be made 



1 The roots are injured by the shock, but an experiment which may be 

 performed to show this without injury requires an elaborate piece of ap- 

 paratus. If care is taken, the tip may be slit lengthwise. In spite of the 

 injury the root will still bend down as the root is not injured. 



