Leaves. 77 



85. Is there any marked difference in position and dis- 

 tribution between leaves on horizontal branches and those 

 on vertical branches ? 



86. Make a longitudinal section with a sharp knife 

 through the middle of a leaf and the branch which bears 

 it, and note its connection with the bark and wood. Re- 

 move another leaf from its stem, with a downward pull, 

 and examine the wound with a lens to see whether the 

 broken ends of the vascular bundles can be made out. 



87. Make drawings to show the course of the veins in a 

 grass or lily leaf, or in the leaf of any monocotyledonous 

 plant ; also drawings of veins in the leaf of a sunflower, 

 castor bean, or other dicotyledonous plant. Bleach any thin 

 leaf by soaking it in strong alcohol until the chlorophyll is 

 extracted (chlorophyll is the green coloring matter in the 

 leaf), and allowing it to lie in a saturated solution of chloral 

 hydrate (see page 381) for several days ; then mount it in 

 a drop of dilute glycerine, and examine under the medium 

 power of a compound microscope in order to note the 

 ultimate branches of the veinlets. 



88. Cover rapidly growing plants or branches of plants 

 so as to keep them dark, and compare the color and size 

 of the leaves which develop in the dark with that of leaves 

 grown under normal conditions of illumination ; compare 

 also the lengths of the internodes formed in the two cases. 

 How long does it take for the blanched leaves to turn 

 green after they have been exposed to the light ? Boil 

 for a short time in water a blanched leaf which has devel- 

 oped in the dark and a green leaf which has grown in the 

 light, and place in alcohol until the green leaf has become 

 colorless ; then place both leaves in a solution of iodine, 

 and note whether one is colored more purple than the 

 other, indicating a greater amount of starch. Sugars, as 



