The Intake of Carbon 223 



weak starch paste (using a piece of starch as large as a lupin 

 seed in 10 cc. of water, and boil a few minutes), then add to this 

 a few drops of an alcoholic solution of iodine and note the intense 

 blue color. This is a common test for starch. Use the iodine 

 test in determining if starch is present in leaves of nasturtium, 

 geranium, or potato, which have been in bright sunlight for a 

 few hours, as suggested in section 112. First extract the chlo- 

 rophyll by alcohol, then stain with the iodine solution. 



Chlorophyll and photosynthesis (starch accumulation). The 

 necessity of chlorophyll in starch-making may be simply shown 

 by using variegated leaves, white and green, of certain varieties 

 of Coleus, or other greenhouse plants of this nature conveniently 

 obtained. From a plant which has been exposed to sunlight 

 several hours select a leaf, outline the white and green areas, and 

 then test for starch as above suggested. Indicate the relation 

 between the occurrence of starch and the areas outlined. For 

 further proof place the plant in the dark a few hours or over 

 night, so that all starch is removed from the leaves, then replace 

 the plant in light and determine if starch is deposited after a few 

 hours, and in what areas. 



Light and photosynthesis (starch accumulation). The obser- 

 vation that leaves are depleted of starch in the dark is alone suffi- 

 cient to suggest that no starch has been formed ; nevertheless, 

 it is instructive to determine if starch is formed in a darkened and 

 aerated area of a leaf, the remaining portion of which is exposed 

 to light. Employ a Ganong aerated box or light screen (Fig. 57) 

 or simpler devices similar in principle improvised for the 

 purpose. After exposing the leaf for a few hours, apply the 

 starch test and describe the conditions and results. 



Obtain two small potted plants, such as Fuchsia, nasturtium, 

 sunflower, or jewel-weed, which shall have been determined to be 

 suitable for starch formation and starch removal in the leaves. 

 Place these in the dark for a few hours or over night, until a test 

 indicates no starch present. Place one in strong light and the 

 other in very weak light, with conditions otherwise as nearly 

 the same as possible. One may be placed under a bell glass and 

 the other under a bell glass covered with manila paper, or with 



