6 - PLB 105 Lab ManualFall 2010 page 6-1 Laboratory 6 Xylem...

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PLB 105 Lab Manual—Fall 2010 page 6-1 Laboratory 6 Xylem The structure of primary xylem tissue and an introduction to secondary xylem tissue are today's topics. Primary xylem matures from procambium, one of the three primary meristematic tissues, and secondary xylem matures from the vascular cambium, which is a lateral secondary meristem. We will return to the topic of secondary xylem in a later lab. Xylem is a complex tissue composed of several cell types. Tracheary elements (vessel elements or tracheids) are always present, and parenchyma cells and fibers are commonly present. To fully understand what you are seeing today, it is important to take your time with each of these exercises. Please work in teams—this will help each team member see all of the material and will foster team discussion, which leads to better understanding. A. Vascular Tissue—Primary Xylem The apical meristem is the origin of primary xylem. The emphasis today is an investigation of xylem tissue cell types, including a comparison of protoxylem and metaxylem in the primary xylem tissue. Primary xylem tracheary elements reveal secondary cell wall patterns that allow for cell extension. As an organ matures, the later-maturing tracheary elements (metaxylem) have a larger relative area of secondary cell wall deposition when compared with the earlier maturing tracheary elements (protoxylem). Typically, the protoxylem tracheary elements develop annular (hooplike) or helical (spiral) secondary cell wall depositions and the metaxylem tracheary elements develop scalariform (ladderlike) or reticulate (networklike) secondary cell wall patterns. As an organ nears maturity, tracheary elements develop the maximum amount of secondary cell wall deposition, leaving only small areas with primary
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6 - PLB 105 Lab ManualFall 2010 page 6-1 Laboratory 6 Xylem...

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