PHOTOSYNTHESIS AND RESPIRATION. 79 



has been properly arranged, the part of the leaf inside the bottle (i.e. 

 in air free from carbon dioxide, which has been absorbed by the lime- 

 water) contains no starch, while the part outside it does. If you do 

 not get this result at first, try the experiment again. This is known 

 as Moll's experiment. Baryta-water or potash may be used. 



114. How is the Volume of the Air affected by 

 Starch-Making? If some living leaves are placed in a 

 closed vessel, how does their absorption of carbon dioxide 

 affect the volume of the air ? Does the volume diminish ? 



* Place a few fresh leaves (from a plant which has been previously 

 kept in darkness for a day) in a glass jar fitted with a bored cork 

 through which passes a twice-bent tube ( J -tube) and let the outer limb 

 dip into water coloured with red ink in a small bottle, as in Fig. 36. 

 Charge the air in the jar with carbon dioxide as before ; cork tightly 

 and seal with plasticine or vaseline. Cause the coloured water to rise 

 a little way in the outer tube, by warming the jar gently and then 

 letting it cool. Mark the level when the preparations are complete, 

 and set the apparatus in good light (not direct sunlight). 



Does the volume of air in the apparatus (and therefore the level of 

 the water) alter at all ? Since changes in temperature affect the level 

 (why ?), it is well to prevent draughts by placing a bell-jar over the 

 apparatus, or to set up a control experiment (omitting only the leaves) ; 

 note the temperatures by hanging a thermometer near the apparatus or 

 passing it through a hole in the cork of the jar. Test the air in the 

 jar for carbon dioxide after a day's exposure to light, by pouring in 

 some lime-water or baryta-water. This experiment shows that when 

 carbon dioxide is absorbed by green leaves, an equal (or 

 nearly equal) volume of some other gas is set free. 



115. How does Starch-Making change the Com- 

 position of the Air ? Have you ever noticed, on a warm 

 sunny day, brisk streams of gas-bubbles arising from plants 

 growing under water in a pond or an aquarium ? 



* (a) Collect some of these submerged water-plants e.g. Water Star- 

 wort (Gallitriche) or Canadian Water- weed (Elodea) and test their 

 leaves for starch. Place them in a large glass vessel (e.g. a bell- jar rest- 

 ing on a wooden support) ; if necessary tie them to a stone to keep them 

 together at the bottom. Are the gas-bubbles given off in darkness ? 

 Cover the vessel with a black cloth and see if the bubbling stops after 

 a short time, then expose the vessel to light again. Water-cress or 

 Mint will do, if the plants mentioned cannot be obtained. 



* (6) What do these bubbles consist of ? Is the gas ordinary air, or 

 oxygen, or carbon dioxide ? We know that water contains dissolved 

 air, and that this dissolved air is richer in carbon dioxide than 



