290 THE BIOLOGY OF THE PLANT 



What is their shape ? Are there any marked 

 variations in shape ? Do they differ much in size ? 

 Does their wall vary much in thickness ? If so, 

 where is it thickest ? "What do the cortical cells 

 contain ? Do you find starch ? Are their con- 

 tents more or less abundant than those of cells 

 lying near the middle of the thallus ? If so, can 

 you explain why ? Examine the layer of paren- 

 chyma which lies between the cortical cells and 

 the medulla. Is this layer sharply defined? 

 What is the general shape of these cells ? Are 

 they well filled with contents ? Is starch present ? 

 Note that the walls of many of the parenchj'ma- 

 tous cells are thickened and marked with circu- 

 lar, oval, or irregular dots, the pits. Study the 

 medulla. What is its color ? Notice how exceed- 

 ingly thick the cell-walls appear. How do the 

 cavities of the medullary cells compare in shape 

 and size with those of the cortical and parenchy- 

 matous cells ? What do the medullary cells con- 

 tain ? Test for starch. Is it present ? Place a 

 section in a drop of fresh water and watch the 

 changes in the medulla. What happens? Do 

 your observations on this section explain the 

 changes seen when a portion of the thallus was 

 placed in fresh water ? Apply a drop of strong 

 sulphuric acid, which dissolves cellulose, to a sec- 

 tion. What happens to the cells of the medulla ? 

 To the cortical cells ? Do the walls of the corti- 

 cal cells, particularly the outer walls, consist of 

 cellulose? To another section apply a drop of 

 Schulze's solution. What change in the walls 

 of the medullary cells? Do you find an inter- 

 cellular substance which is not cellulose? 



