4 o THE VEGETATIVE ORGANS 



adjust itself to the changing conditions. The power of 

 response and adjustment is the most characteristic feature 

 of life, and it is important that we should pursue the 

 subject a little further. 



Contact stimulus. In an ordinary soil, it will commonly 

 happen that roots will meet with obstructions, such as 

 stones. Under these circumstances, how will they behave ? 

 Take a wide-mouthed bottle, half-filled with stones or frag- 

 ments of broken plant pots, moistened with a little water. 

 Then attach two or three seedlings to the cork, and suspend 

 them with the radicles directed downwards into the bottle. 

 Notice what happens as they come into contact with the 

 hard fragments. The roots turn away, escaping the injury 

 which would result if the tip were forced against a solid 

 object. 



The sensory region of the root. This shows that some part 

 of the root must be sensitive, and the following experiments 

 determine the sensory region of the root. 



Take four seedlings of the Broad Bean [a to d) with 

 radicles about i\ inches long, and treat them as follows : 



i. Place seedling a horizontally on moist coco-nut fibre. 



2. Take a razor and cut off one-sixteenth of an inch from 

 the tip of the radicle of seedling b and place it alongside a, 



3. Place seedling c, uninjured, on its side for an hour, 

 then cut off the tip as in b and lay it horizontally on the 

 moist fibre. 



4. Place seedling d on its side for a day until its tip has 

 curved downwards, then cut off the tip as with b and c, 

 but place the seedling with the root pointing downwards. 



Allow the seedlings to grow and carefully compare the 

 results. 



a turns downwards ; b grows, but does not bend ; c bends 

 as in a ; d does not turn downwards, but continues to grow 

 horizontally. 



If the seedlings are allowed to grow under favourable 



