82 THE VECxETATIVE ORGANS 



Watch the effect of the iodine, and note that the parts 

 which have been kept in the dark remain colourless or 

 are merely stained yellowish brown, while those which 

 were exposed to light take on a blue-black colour (Fig. 51,2), 

 indicating the presence of starch, which appears only 

 in those parts to which light had access. 1 



Conditions necessary for the formation of starch. A 

 similar test should be made with leaves of a variegated 

 Geranium, one with white patches on its leaves being 

 selected. The leaves need not be covered, but after exposing 

 the plant to sunlight as in the previous experiment, test 

 for starch by the same methods. It will be seen that 

 starch is formed only in the parts that are green. 



With a Geranium plant that has been kept for a day 

 in the dark arrange an experiment as shown in Fig. 52. 

 Place a little caustic potash solution in the bottom of 

 a bottle, tilt it as in the figure and turn into it a leaf of 

 the plant, taking care that the leaf does not touch the 

 liquid. Close the bottle with a split cork perforated to 

 admit the petiole without injuring it, carefully seal the 

 cork with vaseline, and then expose the whole to sunlight 

 as before. 



Now consider the following points : What effect will 

 the caustic potash have on the air in the bottle ? What 

 changes take place in the composition of the air by the 

 action of a gieen leaf ? Test the leaf as above and deter- 

 mine whether starch has been formed under the conditions 

 of this experiment ? We have seen that plants absorb 

 oxygen from the air and give off carbon dioxide ; this 

 is the process known as respiration or breathing ; also 

 that a green leaf exposed to air containing carbon dioxide 



1 If it is necessary to carry out these experiments during very 

 dull weather, satisfactory results can be obtained by exposing the 

 plants to artificial light, care being taken not to injure the plant 

 by heat. 



