102 APPLIED BIOLOGY 



oxide to enter the spaces of the leaves, whence the carbon 

 dioxide passes into the cells with chlorophyll (Fig. 30). 

 Water reaches these cells by osmosing from near-by veinlets, 

 which receive water from the roots through the stem. 



100. Photosynthesis. That light is necessary for 

 carbohydrate-making (sugar and starch), the following 

 experiment shows. Since the process is a synthesis or a 

 combining depending upon the action of light, it is commonly 

 termed photosynthesis (meaning, combining by light) . Briefly, 

 it is starch-making or sugar-making in green leaves exposed 

 to light. 



Test for Starch. (D) Put a small quantity of corn starch in a 

 tube with water, add a few drops of iodine solution (crystals of I in 

 70 per cent alcohol). Note the blue color. Try effect of iodine on 

 powdered sugar in water. If we had time, we might test in the same 

 way all the various substances found in plants and animals ; 

 but we must accept the records of science that no one has yet found 

 any other substance which with iodine solution gives this peculiar 

 blue color seen in starch. Hence we use iodine as a test for the 

 presence of starch in animal and plant substances. Sometimes 

 iodine is mixed with chloral hydrate in order to make it stain 

 more. 



Starch Formed in Leaves in Light. (D) Take two potted plants 

 (bean, nasturtium, or other convenient plants), leave in a perfectly 

 dark room (or box) one day, and on the morning of the second 

 day take one of the plants from the dark and set in strong sunlight. 

 Near the close of the day take leaves from both plants, dip into 

 boiling water for a minute, and place in bottles (labelled "dark," 

 "light") containing strong alcohol. When the class meets again, 

 note that the green color (chlorophyll) has been extracted by the 

 alcohol ("bleached"). Take a leaf from each bottle, rinse in water, 

 and place in a beaker or a small saucer containing some iodine solu- 

 tion (or better, use saturated solution of chloral hydrate mixed with 

 enough iodine solution to color). In which leaf does the iodine 

 test show presence of starch? Remembering that the two leaves 

 have been treated exactly alike except that one came from a plant 

 left all day in the sunlight, while the other remained in the dark, 

 state your conclusion as to the importance of sunlight in starch- 

 formation. 



