148 SCOTCH PINE. 



quite as large as the bordered ones of the trach- 

 eides. By focusing carefully the walls of these 

 cells may sometimes be seen in section. 



19. The cambium. Note the shape and contents of the cells. 

 There is sometimes difficulty in discovering the end 

 walls of the cambium cells. It can be obviated some- 

 what by examining a section which has lain in glycerine 

 for a few hours. Notice particularly the delicacy of 

 the walls. 



20. The sieve cells. Study 



a. Their shape and arrangement. 



b. The markings on their walls ; round or oval areas 

 of fine perforations, looking like minute specks. 

 Note their arrangement ; compare with that of the 

 bordered pits on the tracheides. 



21. Thz phloem parenchyma ; note length and contents of 

 the cells. 10 



22. The cortical parenchyma. 



a. Study the shapes and contents of the cells. 



b. Notice here and there cells which seem to have 

 been divided by a partition, the pair still retaining 

 an oval shape. 



c. The large intercellular spaces. 



23. The medullary rays. Study their cells in the cambium 

 and sieve cell regions. 



24. The resin ducts. (Their longitudinal structure may be 

 studied either in the longitudinal or transverse section 

 of the stem, the latter usually showing a longitudinal 



10 Difficult to distinguish without staining with methyl blue or chlor-io- 

 dide of zinc. 



