1.
STRUCTURE OF XYLEM
1. TRACHEIDS
2. TRACHEA OR VESSELS
3. FIBRES CALLED XYLEM FIBRES OR WOOD FIBRES
4. PARENCHYMA, REFERRED TO AS XYLEM OR WOOD PARENCHYMA
• Of all parenchyma is living while all other components are dead.
Tracheids
Trachea or vessels XYLEM OR WOOD PARENCHYMA

2.
1.Tracheids
• A tracheid is a very much elongated cell
occurring along the long axis of the organ.
• The cells are devoid of protoplast and hence
dead.
• A tracheid has a fairly large lumen without any
contents and tapering blunt or chisel-like ends.
• The end walls usually do not uniformly taper in
all planes.
• Tracheids are round or polyhedral in cross
section. The wall is hard, moderately thick and
usually lignified

3.
 The secondary walls are deposited in different manners,
so that the tracheids may be annular, spiral, reticulate,
scalariform or pitted.
 Bordered pits are most abundant. Through these pits
they establish communication with the adjoining
tracheids and also with other cells, living or non-living.
Pitted
Spiral
Annular

4.
 Tracheids occur in both primary as well as secondary xylem.
 A typical fibre differs from a tracheid in more pronounced thickening of the wall and
correspondingly much smaller lumen, as well as reduction in the size of the pits.
 An intermediate type of cell element, called fibre tracheid is found in some plants.

5.
2.Tracheae or vessels
These are long tube-like bodies ideally suited for the conduction of
water and solutes.
Short cell, cell are shorter than tracheids
Cell become dead at maturity.
A trachea or vessel is formed from a row of cylindrical cells
arranged in longitudinal series where the partition walls become
perforated, so that the whole thing serves like a tube.
Perforations are commonly confined to the end walls, but they may
occur on lateral walls as well. The walls undergoing these
perforations are called perforation plates, which are of two types –
multiple and simple

6.
• Multiple perforation plate - In primitive plants the
end walls between the cells do not completely
dissolve, but the openings or perforations remain
either in more or less parallel series like bars called
scalariform perforations, or in the form of a network
known as reticulate perforation or a group of circular
holes called foraminate perforations.
• Simple perforation plate – In advanced plants, the
dissolution of the end wall is more or less complete
and the perforation occurs in the form of a large
circle.
Scalariform perforation
plate

7.
 The vessels are considerably long bodies. For e.g. in ash plant (Fraxinus
excelsior) of family Oleaceae vessels have been reported to be as long as 10ft.
Vessel elements are typically found in angiosperms (flowering plants) but
absent from most gymnosperms such as conifers.
 Vessel elements are the main feature distinguishing the "hardwood" of
angiosperms from the "softwood" of conifers.
Vessels are absent in most Pteridophytes except Pteridium and Selaginella and
Gymnosperms except Gnetales.
 They are present in all Angiosperms except some members of order Ranales.
They are also absent in parasites and aquatic plants and secondary xylem of
monocots (secondary growth is absent in many monocotyledons)
 Vessels are the most important members of the xylem as they are primarily
adapted for easy transport of water and solutes and secondarily for mechanical
support.

8.
3.Xylem fibres
• These are the sclerenchymatous cells which remain associated
with other elements of xylem and provide mechanical support.
• They are very much elongated and dead cells with thick
lignified walls.
• Xylem or wood fibres are mainly of two types: fibre tracheids
and libriform fibres.

9.
4.Xylem parenchyma
• Living parenchyma is a constituent of xylem in most plants. These cells are meant for storage of starch, fats and other
matter like crystals and tannins.
• In primary xylem they remain associated with other elements and derive their origin from the same meristem.
• In secondary xylem parenchyma occurs in two forms: xylem parenchyma or wood parenchyma, where the cells are
somewhat elongated and lie in vertical series attached end on end and ray parenchyma, where the cells occur in radial
transverse series.

10.
• Based on their association with vessels xylem parenchyma is classified as: Apotracheal, not in contact
with vessels and Paratracheal, in contact with the vessels.
• Parenchyma is abundant in the secondary xylem of most of the plants except few conifers like Pinus,
Taxus and Araucaria.