The Structures and Processes of Stems 123 



secondarily it is due to changes in the walls themselves 

 (lignification) . 



Annual rings 



Tracheae 



Pith 



Pith 



ravs 



FIG. 71. Block of oak wood magnified to show the arrangement of the various 

 tissues which produce the patterns on polished wood surfaces. (Diagrammatic.) 



Mechanical tissue is found on both the water-conducting 

 and food-conducting sides of the bundle. On the food-con- 

 ducting side it lies outside the food-conducting tissue, and is 

 made up of long, exceedingly slender, nearly solid, spindle- 

 shaped cells. These cells are called bast fibers, and the tissue 



