WORK OF THE ROOT 



43 



will turn away from the light towards the shaded 

 side. Thus Ivy roots are probably not merely growing 

 towards the moist surface, but also away from the sun- 

 light. The sensitiveness of growing organs to sunlight 

 is termed heliotropism (Gr. helios = sun), but as roots 

 usually turn away from, and not towards, the light 

 they are said to be negatively 

 heliotropic. 



Our experiments have shown 

 that roots are sensitive to a 

 number of different stimuli, 

 namely, gravity, contact, water, 

 and light, and that in response 

 to these stimuli roots grow in 

 a definite manner. We will now 

 determine the important fact 

 that roots are sensitive only in 

 the presence of oxygen. 



Necessity for oxygen. Take 

 a wide-mouthed bottle fitted 

 with a good cork or stopper, 

 and fill half of it with water that 

 has been boiled so as to expel 

 the dissolved air. Select two 

 seedlings of the Pea with radicles 

 about an inch long. Take a board 

 (which has been previously boiled 

 to destroy mould-spores) and pin the seedlings to it in 

 such a way that the radicles are directed horizontally 

 as in Fig. 20. Place them in the bottle, submerging 

 one in the water (a), and placing the other well above 

 the water in the air (b) ; then close the bottle with 

 a stopper. Ensure that both seedlings have their radicles 

 directed downwards and place them horizontally only 

 when putting them into the bottle. Leave them for 



Fig. 20. Experiment to 

 show that the root is 

 not sensitive in the ab- 

 SENCE of Oxygen. a, 

 seedling in boiled water ; 

 b, seedling in air. 



