74 PHOTOSYNTHESIS AND RESPIRATION. 



* Keep a plant in darkness until the leaves no longer show starch when 

 tested with iodine, then cut off several leaves and set them in tumblers 

 or bottles of water ; the stalk should dip into the water. Set some in 

 sunlight, the others in darkness, and after two days decolorise and test 

 with iodine the two lots of leaves. Those kept in darkness show no 

 starch, but starch is found in those that have been exposed to light. 

 This means that starch is made by leaves when in the light, but not 

 when in darkness. 



109. Is Light required for Starch-Making? The 



preceding experiments show that starch is found in leaves 

 that have been exposed to light, whilst it is absent from the 

 leaves of the same plant after it has been kept in darkness 

 for a day or two. This suggests (1) that light is required 

 for starch to appear in the leaf, (2) that the starch formed in 

 the light disappears in darkness. 



* Set some plants e.g. Broad Bean, Primrose, Tropaeolum in pots in 

 darkness for at least a whole day. In the morning take off some of the 

 leaves, decolorise them, test with iodine ; they give only a light brown 

 colour, starch being absent. Choose a day with sunshine or good dif- 

 fused light for these experiments : 



(a) Let some of the leaves on the plant remain untouched. 



(6) Fix a strip of tinfoil across the leaf, on both sides (above and 

 below) ; or pin to the leaf two flat slices of cork opposite each other on 

 the two sides ; or stitch to the upper side of the leaf a piece of black 

 paper or cloth. The object in each case is, of course, to exclude light 

 from a portion of the leaf. 



(c) With a knife or scissors, cut the words "starch," "light," your 

 initials, etc. , in capital letters out of a series of cards ; fasten each card 

 to a broad leaf (a Primrose plant, dug up and grown in a pot, or in a jar 

 of water, answers well), and, after letting the plant stand for a full 

 day in darkness, expose it to light for several hours, then decolorise 

 and test. In this way we can get what might be called a " starch, 

 print," from analogy with a photographic print. 



A beautiful modification of the experiment is to place a "sharp" or 

 " hard " photographic negative (i.e. one with sharp distinctions of light 

 and shade) over the leaf and expose to light. 



In each case expose the plant to light from morning till late after- 

 noon, then take off the leaves which have been treated in these various 

 ways, tying to each a label, decolorise them and test with iodine solu- 

 tion. Make notes of your observations and conclusions as to the 

 conditions under which starch is made by leaves. After being tested 

 with iodine, the leaves may be preserved in alcohol and thus made 

 colourless again, and on being soaked in hot water will again give the 

 starch reaction with iodine. The same leaves can therefore be 



