Xylem is a complex plant tissue that transports water and minerals. It is composed of tracheary elements like tracheids and vessel members, as well as xylem parenchyma and fibers. Primary xylem develops early in a plant and secondary xylem develops later from vascular cambium. Vessel members transport water more efficiently than tracheids due to perforation plates and evolutionary specialization of pitting and element shape. Xylem parenchyma and fibers provide support. Tyloses are outgrowths from parenchyma cells that block vessel lumens.

2.  Use
in the identification of vascular plants
rather than phloem
 Complex tissue
◦ consists of several different types of cells,
living and nonliving

3.  Primary xylem
◦ from the procambium
◦ differentiates in the primary plant body
 Secondary xylem
◦ from the vascular cambium
◦ differentiates in the secondary plant body

4.  Tracheary elements
◦ Tracheid
◦ Vessel element
 Xylem parenchyma
 Xylem fibers

5.  have lignified secondary walls
 cells without protoplast
◦ Tracheids – imperforate elements,
elongated
◦ Vessel members – with perforations
 On the end walls called perforation
plate

6. 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

9. Formation of a vessel
 originates
from a longitudinal series of
meristematic cells
 mayor may not elongate but expand
laterally
 thickening of the intercellular
substance

10. Formation of a vessel
 develop 20 wall in a characteristic pattern
 swollen regions of the primary wall break
down
 death of the protoplast

12. 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

15. 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

17. 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

19.  Have
thick walls and reduced borders in
comparison to tracheids
 Kinds
◦ Fiber-tracheids
◦ Libriform fibers
 Retention
of protoplast in evolutionary
advanced forms

22.  In the primary xylem
 In the secondary xylem
◦ Axial parenchyma
 Fusiform parenchyma
 Parenchyma strand
◦ Ray parenchyma
 Procumbent ray
 Upright ray

24.  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