THE ROOTS 39 



43. The tips of primary or main roots point 

 downward. — Germinate some peas or beans, as in 

 § 34, and when the first roots are two or three 

 inches in length, thrust a pin through the seeds and 

 fasten to a piece of wood or cork with the tips 

 pointing directly upward. Float the cork or wood 

 in a saucer of water and cover with an inverted 

 tumbler. Examine two to four hours later. The 

 tip will generally be found to be pointing downward, 

 having curved near the apex. This behavior is due 

 to its sensitiveness to gravity. The root tends to 

 place its axis in a position in which the tip is 

 directed toward the centre of the earth. This move- 

 ment is not caused by the weight of the root, and it 

 does not bend like a piece of soft wax, as may be seen 

 if you attempt to bend it back to its original posi- 

 tion. It breaks in consequence of such forcible bend- 

 ine. The curvature of the root is due to the action 

 of certain definite parts designed to do such work. 



44. The tips of branches of the main roots are 

 directed horizontally. — The downward growth and 

 extension of the first roots formed by a seedling is 

 a necessity for almost all plants. After the main 

 root has bored down into the soil, it finds the food 



