158 



GREEN PLANTS AS FOOD MAKERS 



H2O 2 

 starch made 



warm alcohol 

 extracts, 

 green color' 



decolori^ecL 



Mi t 



iodine 



What effect does 

 sunlight have on green 

 leaves ? Describe the 

 experiment which will 

 prove this. 



interferes with the blue color of the starch test). 

 A test with iodine shows that starch is present 

 only in the portions of the leaves exposed to sun- 

 light. From this we infer that the sun has some- 

 thing to do with the amount of starch found in a leaf. 



The necessity of air for making carbohydrates 

 may easily be proved by experiments. For if 

 parts of several leaves on a plant are covered with 

 vaseline, they will be found to contain no starch, 

 while those parts of the leaf without vaseline, but 

 exposed to the sun and air, will contain starch. 

 The part of the air used in carbohydrate-making 

 is carbon dioxide, which is present in the atmos- 

 phere in very small amounts. 



Air is necessary for the process of making sugar 



and starch in a leaf, not only because carbon 



dioxide gas is absorbed but also because the leaf 



is alive and must have oxygen in order to do 



its work. It takes this oxygen from the air. 



Practical Exercise 8. Explain wiry some plants do so 

 poorly in the house. Why are trees in cities often so 

 sickly? 



Demonstration 10. To show the need of chloro- 

 phyll for making carbohydrates. 



Place a plant with variegated leaves, as Coleus, in 

 sunlight for an hour or two. Test several leaves with 

 iodine after removing the chlorophyll with methyl 

 alcohol. Do all the leaves show presence of starch? 

 Do all parts of the variegated leaves show starch? 

 Why is chlorophyll necessary? 



Demonstration 11. To show the need of carbon 

 dioxide for making carbohydrates. 



Place a green plant in a wide-mouth jar which 

 contains carbon dioxide gas. Place the jar in bright 

 sunlight. Place another plant in a jar in which 

 carbon dioxide is removed by means of soda lime 

 (see diagram) . After 24 hours test leaves from both 

 plants for starch. Results ? 



Chlorophyll necessary for making carbohy- 

 drates. In the palisade layer of the leaf, we find 

 cells which are almost cylindrical in form. In 



