142 General Botany 



cheids) are thick-walled and spindle-shaped, with numerous thin 

 places, or pits, in two of the walls. Because of this structure, the 

 stem retains its rigidity and still permits the ready passage of 

 water and mineral salts. 



The stems of some conifers, such as pine, spruce, and fir, have 

 resin ducts distributed more or less irregularly in the wood. In 

 cedar, hemlock, sequoia, cypress, and arbor vitae, resin ducts are 

 absent. Resin 'ducts are not tubes Hke the tracheae of dicot stems, 

 but are intercellular spaces in which resin accumulates. 



