WORK OF THE ROOT 



4i 



conditions a new tip is formed and grows downwards as 

 in an uninjured root. We thus see that, though able to 

 grow in length, seedlings b, c, and d had lost their sensitive- 

 ness, and that the last one-sixteenth of an inch includes 

 the region which is able to receive and respond to a 

 stimulus. This is known as the sensory region of the root. 

 The stimulus of water. Hydrotropism. To pursue the 

 subject of root sensitiveness further, perform the following 

 experiment. Obtain a shallow box, remove the bottom, 

 and replace it by wire gauze (Fig. 19). Fill the box with 

 wet coco-nut fibre and sow in it a number of peas. Tilt 



Fig. 19. Experiment to show that Roots are sensitive 

 to the Stimulus of Water. 



the box at an angle of 45 and protect the bottom from 

 strong light. As the seeds germinate, the roots, owing to 

 the shallowness of the box, soon grow through the gauze 

 into the air. Note the behaviour of these roots. We see 

 that they cling to the surface and may even bend back 

 and grow upwards into the wet fibre. From previous 

 experiments we should expect the roots to grow vertically 

 downwards in response to the stimulus of gravity. What 

 other force is now operating to draw the roots away from 

 the vertical direction ? We see that the attraction of water 

 is, under these circumstances, more powerful than that of 

 gravity. This tendency of roots to turn towards or be 



