THE FOOD OF THE PLANT 



55 



Experiment 35 



Aim.^^-To find out if the absence of carbon-dioxide 

 affects the formation of starch in the leaf. 



Method. — Two potted plants are put into the dark 

 for several days until a leaf, when submitted to the 

 iodine test, gives no starch reaction. 



They are then brought again into the light and 

 placed imder two large bell-jars. Each bell-jar is fitted 

 with a cork, through which is placed a piece of glass- 

 tubing bent twice at right angles and having a U-tube 

 fixed at its free end (Fig. 27). 



In one case the U-tube is filled with soda-lime and 



Fig. 27 



a dish of strong caustic potash solution is put under 

 the bell-jar. The caustic potash absorbs the carbon- 

 dioxide already within the bell-jar and the soda-lime 

 absorbs that from the new air which enters the jar 

 through the tube. The plant is thus kept in an atmos- 

 phere deprived of carbon-dioxide. 



The second plant is used as a control. In this case 

 the dish of caustic potash solution is omitted, and 

 small pieces of pumice are put into the U-tube instead 

 of soda-lime. 



