2. XYLEM
▪ The word xylem is derived from the Greek word ‘Xylos’ meaning wood.
▪ This term was proposed by Nageli(1858)
▪ It is one of the conductive tissue in plants.
▪ A complex tissue, composed of many types of cells.
▪ Main function : Conduction of water & minerals.
▪ Additional function : Provide mechanical support.
▪ 2 types; Primary xylem & Secondary xylem.
3. ▪Primary xylem
▪ Formed during primary growth of the plant.
▪ Derived from Pro-cambium (apical meristem).
▪ Consist of 2 parts; Protoxylem & Metaxylem.
▪ Protoxylem: First formed xylem, contains fewer amount of tracheary elements
& more amount of Parenchyma.
▪ Metaxylem: Differentiate only after protoxylem develop. Contain more
tracheary elements than Parenchyma. They are functional in the plants
without secondary thickening throughout their life cycle. In plants with
secondary thickening, metaxylem are replaced by secondary xylem.
4. ▪Secondary xylem
▪ Formed from vascular cambuim(lateral meristem)during secondary growth.
▪ Its walls are thickened due to the deposition of lignin thereby rendering
mechanical support.
▪ Form annual rings.
▪ Differentiation in to sap wood and heart wood found in large woody trees.
▪ Usually vessels remain plugged with tyloses.
▪ Secondary xylem Provides additional structural support and water conduction
tissues in shrubs and trees.
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8. COMPONENTS
Tracheids
▪ One of the fundamentalcell type in xylem.
▪ Elongated tube-like cell with both ends tapering.
▪ Walls thick in some palaces and thin in others.
▪ They serve both as water conducting & strengtheningcells.
▪ Non living at maturity.
▪ Mature cell is empty without Protoplast.
▪ Cells with secondary cell wall which is highly lignified.
▪ Cells are angular or polygonal in cross-section
▪ Ends of tracheids of secondary xylem is chisel like.
▪ Lateral walls and end walls are provided with pit pairs(facilitate lateral conductionof water)
▪ Water and mineral passage take place through pit membrane.
9. Vessels
▪ Elongated tubular structurewith wide lumen.
▪ Cheief conducting element in xylem.
▪ Shorter cells than tracheids.
▪ Cells are non living at maturity.
▪ Cells deviod of Protoplast at maturity.
▪ Cells are arranged as series in end to end, parallel to the long axis of organ in which they
occur.
▪ Components of vessels are called vessel segments or vessel elements.
▪ Mainly present in xylem of angiosperms.
▪ Secondary thickening similar to that of tracheids.
▪ Main function: conduction of water and nutrients, mechanical support (thick lignified
cell wall)
▪ Vessels believed to be originated from tracheids.
10. ▪ Tracheary cells are divided into several types according to the types of
thickening of walls.
▪ Annular tracheary cells – in the form of rings
▪ Spiral tracheary cells – spiral thickening
▪ Pitted tracheary cells – walls which are uniformly thickened except for thin
places in the form of pits.
▪ Scalariform tracheary cells – ladder like thickening
▪ Reticulate tracheary cells – in the form of network
11. Xylem fibres
▪ Dead cells, no protoplast at maturity.
▪ Sclerenchymatous
▪ Long, slender, pointed cells with greatly thickened walls and few small pits.
▪ Cells with very thick lignified secondary cell wall.
▪ Obliterated central lumen.
▪ Main function: provides mechanical support.
▪ 2 types; Fibre tracheids & Libriform fibres.
▪ Longer than tracheids
12. ▪Xylem Parenchyma
▪ Living componentof xylem
▪ Cells with cytoplasm and prominent nucleus.
▪ Thin cellulosic cell wall with simple pits.
▪ Lignified secondary cell wall absent.
▪ Store starch, oil, and ergastic substances.
▪ 2 types; Axial Parenchyma & Ray Parenchyma.
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14. SECONDARY XYLEM ONTOGENY & STRUCTURE
▪ Secondary xylem are type of xylem produced during secondary growth of
angiosperms & some gymnosperm.
▪ The cambium ring cuts off new cells on inner side are gradually modified into xylary
elements called secondary xylem.
▪ Highly specialised vascular tissues characterized by the presence of heavily lignified
secondary cell wall composedof mainly cellulose, hemicellulose and lignin.
▪ Vascular cambium composes of 2 type of stem cells;
(a) fusiform initials (b) Ray initials
▪ The elongated fusiform initials differentiate into axially oriented wood cells (fibres,
vessels, tracheids, axial parenchyma) – ensuring water conduction and mechanical
support.
▪ The nearly isodiametric ray initials give rise to transversely oriented ray
parenchyma ensuring traverse conduction and nutrient storage.
15. ▪ The xylem mother cells cut off by cambium may develop into permanent xylem
elements without further division or may divide once or several times before mature
cells are formed.
▪ Tracheids are formed directly from the xylem mother cell by increase in radial
diameter and in length, thickening of wall and loss of protoplast.
▪ Wood parenchyma cells are formed by the transverse division of the mother cell into a
number of segments and by subsequent radial enlargement and thickening of walls of
the segments.
▪ The parenchymatous nature of ray is likely achieved by ethylene produced by
developing tracheary elements & transported in the rays, inhibiting the differenriation
of non parenchymatous cells in the rays and ensuring their balanced distribution in
wood tissue.
16. APOTRACHEAL & PARATRACHEAL PARENCHYMA
1.Apotracheal parenchyma
▪ “Trachea,” which refers to a tube or pipe (in this case, a wood pore). Combine this with
the Greek prefix “apo,” which means away from or separate.
▪ Apotracheal refers to parenchyma cells that occur separate from the pores.
▪ Mainly classified into 3 types
▪ Apotracheal parenchyma can occur as single scattered cellscalled diffuse parenchyma.
▪ These cells are too small to be seen without a microscope.
▪ In some wood species, several apotracheal parenchyma cells are joined or aggregated
together, forming thin but visible tangential lines. This formation is known as diffuse-in-
aggregates parenchyma.
▪ When horizontal (tangential) bands of parenchyma occur as diffuse in aggregates known
as banded parenchyma.
21. 2.Paratracheal parenchyma
▪ The counterpart to apotracheal parenchyma is paratracheal parenchyma.
▪ The Greek prefix “para” means beside or near.
▪ Parenchyma that occurs in association with the wood’s pores.
▪ It exhibits a much wider range of patterns and variations.
▪ Mainly classified into 4 types.
▪ The most basic paratracheal parenchyma formation is a ring or circle of
cells surrounding the pore, which is termed vasicentric parenchyma.
▪ The words “vase”suggesting a vessel or pore and “centric“, which simply
indicates that the parenchyma is centered around the pore.
23. ▪ Another form of parenchyma that is closely related to vasicentric is Aliform
parenchyma which literally means “wing-shaped.”
▪ There are actually two primary variants of aliform parenchyma: the first
is winged, where short appendages or wings of parenchyma extend from
one or both sides of the pore.
▪ The second variant of aliform is lozenge, where the parenchyma
surrounding the pore takes on a diamond or elongated oval shape.
Ramin: aliform-winged Merbau: aliform-lozenge
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25. ▪ Another formation is Confluent parenchyma. This occurs when the parenchyma
is so extensive that it extends outward and makes contact with the parenchyma
from neighboring pores.
Marblewood: confluent
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27. ▪ When horizontal (tangential) bands of parenchyma occur as extensions of
aliform or confluent parenchyma it is called banded parenchyma.
▪ Banded parenchyma can be in continuous bands, or it can occur in interrupted
or discontinuous bands.
▪ The bands can be very thick constituting over half of the wood’s overall volume
in some species or they can be very thin and hardly visible with a hand lens.
▪ The bands can be very numerous and evenly spaced, or they can be very sparse
and sporadic.
29. ▪ It consist of compact mass of thick walled cells so arranged as to form 2 systems-
1. A longtitudinal (vertical) – Axial system
2. A transverse radiating system (run horizontally) – Ray system
▪ Longtutudinal system consists of elongate, overlapping & inter-locked cells of
tracheids, fibres, vessel elements & a longtitudinal row of parenchyma cells.
▪ It moves water and minerals up the stem.
▪ Secondary xylem traversed by radial system of xylem rays (in a radial direction)
▪ Ray system extends at right angle to tracheary elements and involved in radial
conduction of water and minerals as well as storage.
▪ Some conifer wood contains resin ducts or canals in both the axial & ray system.
STRUCTURE
30. ▪ Secondary xylem in general consist of the kind of cells as those of primary xylem,
but types of vessels, tracheids fibres are often quite different.
▪ Vessels and tracheids are most abundant and usually shorter, wider and thick
walled than that of primary xylem
▪ Rich in xylem fibres than primary xylem.
▪ Mostly vessels are pitted.
▪ Annular & spiral tracheids and vessels altogether absent.
▪ Xylem parenchyma cells are long and fusiform, but sometimes short. They are
living cells and usually meant for storage of food materials.
▪ Tannins and crystals are frequently found in these cells.
▪ Fibers possess thick walls and bordered pits.
31. VASCULAR RAYS
▪ As the radial width of the stele increases, the distance from phloem to xylem
increases and resulting need for better conduction of water radially from xylem
to phloem & cambium and food from phloem to cambium and to the living wood
parenchyma is met with by the production of vascular rays.
▪ Certain cells of the cambium ring instead of giving rise to wood and phloem
elements produce parenchyma cells which form narrow medullary rays traversing
radially the secondary xylem.
▪ They are somewhat elongated and apparently fitted for radial conduction of food
materials.
▪ All vascular rays are initiates by cambium and once formed are increased in
length indefinitely by cambium.
▪ Commonly called medullary rays or pith rays.
32. ▪ These radial rays may be best called vascular rays as the rays are of vascular
tissues partly of xylem and partly of phloem.
▪ The part of vascular ray which is in the xylem is spoken off as xylem rays or
wood rays & that of phloem as phloem rays.
▪ Xylem rays traverse in secondary xylem and establish communication with
living cells of vascular tissues.
▪ Extend radially in secondary xylem.
▪ Originate from ray initials.
▪ Xylem rays runs as a continuous band to secondary phloem through cambium
forming a continuous conducting system.
▪ Xylem rays help in exchange of gases also aid in conduction of water and food
from phloem to the cambium and xylem parenchyma.
33. FUNCTIONS
▪ Conduction of water, nutrients and minerals from root to leaves.
▪ Provides mechanical support.
▪ Storage of food and other materials.
▪ Ray parenchyma forms tyloses which forms ergastic substances.
