WORK OF THE SHOOT 



81 



the cultivated Geranium serve very well. Keep a plant 

 in the dark for a day, then partially cover two or three 

 of the leaves while still on the plant with black paper, 

 tin-foil, cardboard, or pieces of cork, as shown in Fig. 51, 1, 

 in such a way that light is excluded from one part of the 

 leaf while the other may be illuminated. Place the plant 

 so prepared in sunlight for several hours or, if more con- 

 venient, on two or three successive days. At the end 



Fig. 51. Experiment to show that Starch is formed in 

 Green Leaves when exposed to Light. 1, leaf covered with 

 opaque paper, from which a cross has been cut ; below is a cork 

 pinned to the leaf to exclude the light ; 2, the same leaf when 

 tested later with iodine solution ; the shaded areas contain starch. 



of a period of several hours' illumination make the 

 following test. Have ready some boiling water ; remove 

 the covered leaves, noting that they are still green, and 

 plunge them at once into the water to kill them. Next 

 place the leaves in alcohol, by which means the green 

 colouring matter is gradually extracted. (This may be 

 hastened by boiling in alcohol, but it must be done very 

 carefully to prevent ignition of the alcohol vapour.) The 

 resulting colourless leaves should be washed in water, then 

 laid out in a shallow dish and covered with iodine solution. 



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