46 EXPERIMENTAL PLANT PHYSIOLOGY 



This is (not easy. The question must be attacked in 

 an indirect manner, and the process of reasoning must 

 be carefully followed, otherwise the value of the experi- 

 mental evidence may be lost. 



A simple substance commonly found in plant-tissues 

 is selected ; and, by experiment, the conditions under 

 which this substance is formed in the plant are de- 

 termined. 



Now, one of the simple substances most commonly 

 found in the tissues of a plant is starch. Potatoes and 

 wheat contain a great deal of starch ; and, as it will be 

 sho'ffTi presently, starch is not confined to those parts 

 of a plant that we use as food. 



Starch is therefore chosen for the investigation, and 

 the conditions under which it is formed are investi- 

 gated. It is necessary to have some simple test by the 

 application of which the presence of starch can always 

 be recognized. 



Experiment 27 



Aim. — To show the effect produced by iodine on 

 starch. 



Method. — Take a small quantity of starch, powder 

 it and mix it into a paste with a little water. Then 

 add to it a few drops of very weak solution of iodine in 

 potassium iodide. 



Next take several substances which are known to 

 contain starch, and others that are known to be with- 

 out it. Suitable substances containing starch are, for 

 instance, a bean seed, a slice of potato, and a starched 

 collar ; as examples of substances without starch, a 

 piece of white chalk or of washing soda may be used. 

 Each of these substances should be treated in turn with 

 the iodine solution. 



Observations. — The starch is turned a purple blue 

 colour by the iodine solution, so also are all the sub- 

 stances which contain starch. Those without it, on 

 the other hand, just take on the brown colour of the 

 iodine solution. 



