WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 175 



covered by a film of air, but the experiment shows that a 

 land-plant cannot make much starch when deprived of 

 free air by being submerged in water. For comparison, 

 set a few of the starch-free leaves with only their stalks 

 dipping into water in a bottle ; expose to light alongside 

 the jar containing the submerged leaves for the same 

 length of time ; then test with iodine. 



(c) Test with iodine the leaves of a submerged Elodea 

 plant that has been exposed to light : starch is present. 

 Place the vessel in darkness until the leaves are starch- 

 free, then place one shoot (A) in a jar of water that has 

 been boiled in order to expel the dissolved gases, and a 

 second shoot (J3) in a jar of ordinary water. Expose the 

 two to bright light, and after some hours test sample 

 leaves of A and B for starch. 



(c?) Tie together two large flat medicine-bottles of 

 exactly the same size. Into one (A) pour some water, 

 into the other (B) place some carbon dioxide absorbent 

 either caustic potash, or freshly-made soda-lime, or baryta- 

 water. On the rim of the neck of each bottle place some 

 plasticine and vaseline, then press down on the two necks 

 the two halves of a large leaf of Tropaeolum or Tobacco 

 Plant (previously kept in darkness so as to be starch- 

 free), and pat over each a glass slip with a weight to keep 

 the leaf down. If Tropaeolum is used, let the stalk of 

 the leaf dip into a bottle of water. The circular portion 

 of leaf over bottle A receives carbon dioxide from the air 

 in the bottle ; the portion over bottle B receives air de- 

 prived of carbon dioxide. Expose to light for several 

 hours, then test the leaf with iodine : on one side of the 

 leaf there will be a circular patch (A) containing starch, 

 on the other side (B) a patch without starch. 



(e) Put a Tropaeolum leaf in a small bottle of water, so 

 that its stalk dips into the water while the blade rests on 

 the neck of the bottle. Pour some caustic potash into a 

 large jar, place the bottle in the jar, cork the jar tightly 

 and make it air-tight with plasticine or vaseline and wax. 

 The leaf is now exposed to air whose carbon dioxide has 

 been absorbed by the potash. Set up a control experi- 



