92 EXPERIMENTAL PLANT PHYSIOLOGY 



Observations. — After a while the tip of the shoot 

 grows through the hole. 



If the method given for Experiment 59 be used, it 

 will be seen clearly that the stem of the shoot places 

 itself in the same direction as that in which the light 

 is falling, while the leaves arrange themselves at right 

 angles to the source of light. 



Inference. — A shoot grows towards the light, the 

 positions taken by the stem and leaves respectively 

 being such as to secure for the leaves the maximum 

 amount of light. 



The way in which the shoot reacts in response to 

 the stimulus exerted by the direction of the rays of 

 light is termed Heliotropism. The shoot grows to- 

 wards the source of light, placing itself in the same 

 line as the rays of light. It is described therefore as 

 heing positively heliotropic. 



Experiment 58 



Aim. — To find out whether light affects the direction 

 of growth of the root. 



Method. — Get a wooden box — a margarine box answers 

 the purpose well, or a chalk box may be used. Take 

 off one of the long sides and fix a piece of glass in place 

 of the cover. Let the box stand on the remaining long 

 side and fill it with earth. 



Place a row of broad-beans close to the glass. (See 

 also the method used in Experiment 59.) 



Observations. — The roots do not grow vertically 

 downwards but disappear from the side of the glass into 

 the darkness of the soil. 



Inference. — A root grows away from the light. 



The direction of growth taken by the root is therefore 

 said to be negatively Itdiotropic. 



