20 Plants and their Ways in South Africa 



{Waacht-en-beeche). Mark the stems in Spring, when they are 

 growing rapidly. 



Direction of Growth of Roots and Stems. — We 

 have seen that the root tends to grow toward the centre of 

 the earth. The side roots extend obHquely in several direc- 

 tions, to be as well placed as possible 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 ? 



Mark the roots of seedlings in the circular 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 a sharp knife cut off the 

 extreme tips of some. • Early the next day notice the difference 

 between 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. 



The Direction of the Stem. — The stem has as strong 

 an upward growth as the root has a downward tendency. 

 Place seedlings that have grown in pots or in the circular 

 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 

 Commelyna 2 



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 more apparent by thrusting into 

 the soil two slender wire rods, one on either side of the stem. 



Growth Curvature affected by Water. — We have 



