52 EXPERIMENTAL PLANT PHYSIOLOGY 



Put two vessels, each containing a piece of Elodea, 

 into the dark, until the plants are depleted of starch. 



Then surround one of the vessels with ice, put a 

 thermometer into each, and place both in a good light. 



Observations. — No starch is formed in the plant 

 contained in the vessel that is surrounded with ice, but 

 it is formed in large quantities in the control experi- 

 ment. 



Inference. — Warmth is necessary for the formation of 

 starch. 



Experiment 33 



Aim. — To find out if the colour of the leaf affects the 

 formation of starch in it. 



Method. — Choose a plant whose leaves are green in 



^^ 



a. ITie variegated leaf that was tested. The green part is shaded. 



6. The leaf alter it had been boiled in methylated spirit. 



c. The leaf after treatment with iodine. The part which turned blue is shaded. 



Fig. 26 



part only. Variegated maple gives very good results, 

 but any variegated leaf can be used. 



In all experiments that depend on the iodine test it 

 is advisable to select thin leaves, as it is difficult to see 

 the colour reaction through a very thick skin. 



From the results obtained in Experiments 29 and 30, 

 it is clear that the afternoon is the best time for the 

 working of this experiment, as then the leaf has had 

 sufficient time to make a good supply of starch. 



