78 Introduction to Botany. 



well as starch, are manufactured within the leaf ; but the 

 starch is more readily demonstrable, and will therefore be 

 considered here and in the discussion as the visible product 

 of the leaf's work. 



89. Make thin cross sections of a green leaf and mount 

 them in a drop of water under a coverglass and study 

 them with a high power of the microscope. Note the 

 rounded green bodies, termed chloroplasts, lining the walls 

 of the cells. Mount other sections under a coverglass in 

 a drop of a saturated solution of chloral hydrate to which 

 has been added sufficient iodine to give it a pale brown 

 color. By this process the leaf will be bleached and 

 rendered transparent, while the starch grains in the leaf 

 will be stained purple. Watch the action of this reagent 

 from the beginning, and note how the chloroplasts are 

 gradually bleached, while the starch grains in them become 

 more and more distinct. Treat in this way sections from 

 a leaf taken just before sunset, and from another leaf from 

 the same plant taken just before sunrise the next morning. 

 Note the relative amounts of starch in the two sections. 

 What conclusions do you reach from the results of your 

 observations ? 



90. Tie a branch of a floating water plant, such as Cera- 

 tophyllum or Myriophyllum, to a glass rod or tube and set 

 in a beaker of water, the glass rod being about an inch 

 shorter than the depth of the water. Invert a test tube 

 of water over the rod and plant, the cut end of the plant 

 being uppermost and extending into the test tube. Prepare 

 two other branches in the same manner, filling the second 

 tube with water which has just been boiled and cooled, 

 and the third with water which has been charged with carbon 

 dioxide by blowing into it for some time through a glass 

 tube ; invert each tube in a beaker containing the same 



