ILLUSTRATIVE STUDIES 73 



other sections directly in chloroiodide of zinc. The walls that 

 turn yellow in this are cutinized and those that turn blue are 

 cellulose. 



2. Select two similar apples and pare one. Weigh them 

 both and hang them up by their stems. After forty-eight hours 

 weigh them again and estimate the percentage of loss of water in 

 each case. 



3. Heat a glass slide quite hot and press the skin of an apple 

 against it. Notice the waxen spot that is left on the glass. 



4. Study cross sections of a marshmallow leaf, for instance, 

 and measure the thickness of the outer wall of the upper and 

 lower epidermis. Draw a few cells from each epidermis to scale. 



5. Make note of what becomes of the epidermis in the stem 

 of Aristolochia as the stem grows older, using the permanent 

 mounts already prepared for the previous chapter. 



6. Strip the epidermis from an Iris leaf or any other leaf from 

 which the epidermis is readily removed and mount it in a drop 

 of dilute glycerine. Count the number of radial walls (walls on 

 edge in the preparation) in one or more squares of the eyepiece 

 scale and estimate the number of these in a square millimeter. 



7. Determine from cross sections of old and young Aristolochia 

 stems where the cork in the old stems comes from. Measure 

 an average cork cell and the thickness of its walls. Draw a few 

 cells to scale. 



Mount a cross section of old Aristochia stem in chloroiodide 

 of zinc, and the walls of the cork cells should turn yellow. Meas- 

 ure the thickness of the entire cork layer. 



8. Cut in elder pith sections from a bit of potato peeling 

 that has been hardened by standing in 95 per cent, alcohol. 

 Study sections in dilute glycerine and chloroiodide of zinc. 

 Compare the cork cells found here with those in Aristolochia. 

 Measure the thickness of the cork layer. 



9. Select two potatoes of about equal size and pare one. 

 Weigh them and hang them up for forty-eight hours; then 

 weigh them again and estimate the percentage of loss of water 

 in each case. Compare these figures with those from the apple. 



