92 



or other substance, which does not allow light to pass through it, 

 and the plant with its partly covered leaf is then placed in 

 bright sunshine for some hours. The leaf is then cut off 

 and killed by being placed a short time in boiling water. 

 It is then placed in warm alcohol and thus decolourised, the 

 chlorophyll passing into the alcohol in solution. The leaf is 

 then placed in a solution of iodine and finally washed with water, 

 when all parts which had been exposed to the light will be found 

 to have turned blue-black, the colour indicating the presence of 

 starch, the manufacture of which was only possible in those 

 parts where the chlorophyll corpuscles were exposed to the 

 light. 



