secondary cell wall thickening

• Annular

  specimen: zea mays

  
• Specimen: Zea Mays

  A) annular

  B) helical

  
• Scalariform

  specimen: Helianthus sp.

  
• reticulate

  specimen: tilia sp

  
• simple pitted

  specimen: cucurbita sp.

  
• bordered pitted

  specimen: pinus sp