Use in the identification of vascular plants rather than phloem

 Complex tissue

◦  consists of several different types of cells, living and nonliving

 Primary xylem ◦ from the procambium ◦ differentiates in the primary plant body

 Secondary xylem ◦ from the vascular cambium ◦ differentiates in the secondary plant body

  Tracheary elements

◦ Tracheid

◦ Vessel element

  Xylem parenchyma

  Xylem fibers

  have lignified secondary walls   cells without protoplast

◦ Tracheids – imperforate elements, elongated

◦ Vessel members – with perforations  On the end walls called perforation plate

Perforation plate  A. Simple perforation plate  B. Multiple perforation plate

 scalariform perforation plate –parallel series of perforation plates

 reticulate perforation plate – net arrangement of perforation plates

 ephedroid perforation plate- circular holes

Formation of a vessel

 originates from a longitudinal series of meristematic cells

 may or may not elongate but expand laterally

  thickening of the intercellular substance

Formation of a vessel

 develop 20 wall in a characteristic pattern

  swollen regions of the primary wall break down

 death of the protoplast

Structure of secondary wall  annular - deposited as rings  helical – deposited as spirals  scalariform – meshes distinctly

elongated transversely  reticulate – deposited as networks  pitted – secondary wall interrupted

only in the pit areas

Types of pitting

 scalariform pitting – elongated transversely and arranged in vertical series

 opposite pitting – circular or oval bordered pits arranged in horizontal pattern

 alternate pitting – bordered pits arranged in oblique series

Phylogenetic specialization

  tracheid is a more primitive element

  vessels evolved from tracheids   from long, narrow, elongated, with tapering

ends to short, wide with transverse end wall elements

  pitting from scalariform to opposite to alternate pitting

 Have thick walls and reduced borders in comparison to tracheids

  Kinds ◦ Fiber-tracheids ◦ Libriform fibers

  Retention of protoplast in evolutionary advanced forms

  In the primary xylem

  In the secondary xylem

◦ Axial parenchyma  Fusiform parenchyma  Parenchyma strand

◦ Ray parenchyma  Procumbent ray  Upright ray

 Outgrowth of a ray cell or an axial parenchyma cell into the lumen of a vessel

 Arises : ◦ in non-functional vessel ◦ as defensive mechanism

in the presence of plant pathogens