PHOTOSYNTHESIS AND RESPIRATION. 77 



112. Is Chlorophyll needed for Starch-Making? 



Have you noticed, in your experiments with leaves of Broad 

 Bean, Tropaeolum, or Primrose, that the veins remain un- 

 stained even when the rest of the leaf is black with the iodine- 

 test ? This in itself shows that starch is only formed in the 

 green parts of the leaf. 



* (a) Try variegated leaves (e,g. varieties of Geranium or Ivy), making 

 a careful drawing of each leaf before it is decolorised and tested with 

 iodine, and observe that only the green parts produce starch. In 

 leaves whose veins are colourless, or nearly so (e.g. Primrose), owing to 

 the absence of chlorophyll above and below them, you will notice that 

 the veins stand out from the rest of the leaf when the iodine-test is 

 applied, by the absence of starch. 



* (6) Grow seedlings (Bean, Pea, etc.) in darkness, then put them in 

 the light for a day and test the leaves for starch. Leave them in the 

 light until their leaves turn green, then test again for starch. 



These results show that the green substance (chlorophyll) is essential 

 for the process of starch-making by leaves. 



113. How is Air concerned in Starch-Making? We 



know from previous experiments that starch can be broken 

 up, by heating it, into carbon dioxide and water, and that 

 carbon dioxide is always present in the air, being produced 

 by all processes of burning when the substance undergoing 

 oxidation contains carbon. We shall see later on that a plant 

 increases in dry weight when grown in a culture solution 

 which contains no carbon, though more than half of the 

 plant's dry weight consists of carbon. 



If you consider these facts carefully you will probably 

 conclude that the carbon in the starch made by the leaf 

 comes from the carbon dioxide in the air. Does the leaf in 

 some way make carbon dioxide and water come together to 

 form starch ? If so, a plant which is exposed to light and is 

 making starch must be taking carbon dioxide from the air. 

 What will be the effect of exposing green leaves, in light, 

 to air containing no carbon dioxide ? 



* (a) Heat in a dry test- tube (1) pieces of laundry-starch, (2) leaves, 

 (3) pieces of stems, (4) pieces of roots, (5) any other plant tissues. 

 Notice that water is given off as vapour which condenses in the upper 

 part of the tube, and that a black charred mass remains (charcoal, i.e. 

 carbon). Fix a J -shaped tube into a cork to fit the test-tube, and let 

 the longer limb dip into lime-water in a bottle or test-tube ; notice that 



