The Structures and Processes of Stems 



123 



secondarily it is due to changes in the walls themselves 

 (lignification). 



Annual rin^s 



Tracheae 





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

 tissues which produce the patterns on poHshed wood surfaces. {Diagrammatic.) 



Mechanical tissue is found on both the waler-conckicting 

 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 hast fibers, and the tissue 



