84 



BOTANY 



Another experiment is the following: Divide the interior of a shallow 

 wooden box into two parts by an incomplete partition. Partly fill the box 

 with sawdust and place the opening in the partition so that it is below the 

 surface of the sawdust. Plant peas and beans in the sawdust on one side of 

 the partition, water very slightly, but keep the other side of the box well 

 soaked. After two weeks, take up some of the seedlings and note the efifect 

 on the roots. 



Water a Factor which determines the Course taken by Roots. — 



Water, as i^ell as the force of gravity, has much to do with the direction 



taken by roots. The smaller 

 roots, especially, are influ- 

 enced by moisture. Mois- 

 ture also determines the kind 

 and abundance of roots on 

 a plant. Water is always 

 found below the surface 

 of the ground, but some- 

 times at a great depth. 

 In order to obtain a sup- 

 ply of water, the roots of 

 plants frequently spread 

 out for very great dis- 

 tances. Most trees, and 

 all grasses, have a greater 

 area of surface exposed 

 by the roots than by the 

 branches. The mesquite 

 bush, a low-growing tree 

 of the American and Mexi- 

 can deserts, often sends 

 roots downwards for a distance of forty feet after water. The 

 dandelion shown in the cut has a greater depth below the surface 

 of the ground than appears above the surface. The roots of 

 alfalfa, a common plant used for hay in the Western states, 

 often penetrate the soil to a distance of ten to twenty feet below 

 the surface of the ground. 



The force of gravity is an exciting cause or stimulus to the 

 growth of plants. The reaction of the plant (or any living thing) 

 to this force is called geotropism. Roots are stimulated by 



Dandelion plant. Photographed by Overton. 



