The term xylem (Greek, xylos = wood) was introduced by Nageli (1858). Chief conducting tissue of vascular plants responsible for conduction of water and inorganic solutes. Components of Xylem
(i) Tracheids :
Tracheids are elongated tube like dead cells (without protoplasm) with tapering ends (tangenital section)
They appear elongated (tangenital section) rectangular or somewhat rounded (radial section), angular or polygonal (cross section) but in some cases they may appear rounded. The walls are hard and lignified but not much thick and enclose a wide empty lumen. The tracheids are long but not as long as fibres. They reach up to 1 mm in length (in some plants they may attain a length of 12 cm or more).
The tracheids of primary xylem developes from pro-cambium whereas those of secondary xylem developes from vascular cambium. In the beginning these cells possess living protoplasm but due to lignification and deposition of thickening materials in wall they become dead at maturity.
Wall thickenings in Tracheids :
Tracheids possess various types of wall thickenings viz. – annular, spiral, scalariform, reticulate and pitted. The protoxylem tracheids have annular (ring-.like) and spiral (helical) thickenings. The metaxylem and secondary xylem tracheids have scalariform (ladder like), reticulate (network) and pitted thickenings. The pits are of two types simple or bordered. The size and number of pits vary greatly in each tracheid.
Functions of Tracheids:
- To conduct water and dissolved mineral elements from roots to the leaves. They are structurally adapted to their specific function.
- They are placed one above the other and also parallel to the long axis. The end walls are perforated by the presence of bordered pits which permit flow of water from one cell to another.
- Also provide mechanical support due to presence of hard and firm secondary walls.
(ii) Vessels (or Tracheae) :
The xylem vessel designates a long tube consisting of a series of drum-shaped cells placed one above the other with their end walls perforated or dissolved. The vessels are. thus, syncytes formed by fusion of cells. Each cell appears circular, oval or sometimes polygonal in cross section with a very wide lumen. They become dead and loose their protoplasm due to deposition of lignified secondary wall.
Vessels of primary xylem develop from pro cambium whereas those of secondary xylem develop from vascular cambium. They can also be derived by dissolution of end walls or pit membranes. Each cell is short but due to cell fusion the tube becomes much longer. It reaches upto 10 cm in length but in some cases it may reach up to 2 m (as in Quercus) or 3 – 6 m (as in Eucalyptus).
Wall thickenings in Vessels:
The most common ones wall thickenings are scalariform, reticulate and pitted. The protoxylem vessels possess annular and spiral thickening which later on become scalariform and reticulate due to deposition of more thickening materials. The metaxylem vessels generally possess simple pits. The vessels of secondary xylem also possess scalariform, reticulate and pitted thickenings. The pits may be simple or bordered.
Perforation plates:
The wall area bearing perforation is called perforation plate. Usually the perforations in vessels are present in the end walls but they may be present in the lateral walls too. The perforations may be simple (single perforation) or multiple (many perforations). The multiple perforations may be arranged in a row (Scalariform perdoration), in the form of net work (reticulate perforation) or in the form of groups of circular holes (foraminate perforation).
(iii) Xylem fibres (or wood fibres) :
The xylem fibres develop from the same meristematic tissue as the other xylem cells. They have lignified secondary walls and narrow cell xlumen. They are usually longer than the tracheids of the same plant and present both in primary as well as secondary xylem.
(iv) Xylem parenchyma :
These are living parenchymatous cells present as component of xylem both in primary as well as secondary xylem.
The parenchyma present in secondary xylem are of two types :
- Wood parenchyma and
- Ray parenchyma.
The wood parenchyma are formed from the fusiform cambial initials whereas ray parenchma are formed from ray initials of the cambium. Both the types have thin walls and living protoplasm. They help in conduction. The main function of xylem parenchyma is storage if fod reserves in the form of starch or fat.
Phloem:
Phloem is the chief food conducting tissue of vascular plants responsible for translocation of organic solutes. The phloem composed of several defferent kinds of cells. The basic components are
- Sieve elements,
- Companion cells,
- Phloem Parenchyma and
- Phloem fobres.
A fifth kind of cell type, the transfer cell, lias Recently been reported from the phloem. Though all the basic components of phloem occur in most of the vascular e&cftmature sieve element has thin or thick, cellulose wall surrounding a highly albuminous and viscous contents. Cytoplasm occurs in the form of thin lining enclosing a big central vacuole. The nucleus, plastids, mitochondria endoplasmic reticulum and dictyosomes are absent. The vacuole is filled with albuminous substances. Presence of leucoplasts has also been reported in the vacuole.
(i) Sieve Elements (Plate and Sieve areas):
There is a presence of sieve plate and sieve areas in sieve elements. Groups of pores present in the walls of sieve elements are called sieve areas. In a sieve cell, these areas are generally located in lateral walls whereas in sieve-tube element. These areas are located in the cross-walls (end walls).
The portion of cross-wall possessing sieve areas is called sieve plate. In most of the angiosperms the sieve plate is single and present in the transverse or oblique end wall (eg. Cucurbita.Nicotiana, etc) It is called simple sieve plate.
In some of the cases, there are many sieve areas in the end walls. These are termed as compound sieve plates.The nucleus disappears in nature sieve elements.
Function of Sieve elements:
Translocation of organic solutes. This is achieved by their special anatomical features. The sieve tubes are syncytes (formed by the fusion of cells) and allow free diffusion of soluble organic substances. The callose also plays important role. Usually the perforations in the sieve plates are surrounded by callose. The callose is soluble and disappears when the solute is dilute so that the solute can pass from one cell to another cell through the pores. The callose reappears and sometimes closes the pores when solute is less dilute stopping the movement.
(ii) Companion cells:
These are living cells, usually always associated with the sieve tubes. In primary phloem the companion cell is formed by unequal longitudinal division or procambial cell. Thus single companion cell is usually associated with each sieve tube element. Sometimes, there may be more (e g. Carrot). Each companion cell is a living cell with thin cellulose cell wall and active protoplast.
It possesses all the important cellular contents viz.- nucleus, mitochondria, endoplasmic reticulum, dictyosomes, plastids, ribosomes, etc. The cytoplasm is compartmentalized due to presence of membrane system. It also contains slime bodies. The common wall between companion cell and sieve tube shows presence of fine pits which are transverse by plasmodesmata. The companion cells are absent in pteridophytes and gymnosperms. They are present in angiosperms (both in monocots and dicots). They assist the sieve tubes in the process of translocation of solutes. .
(iii) Phloem Parenchyma:
These are living parenchymatous cells present as component of phloem. The cells are elongated with rounded ends and possess cellulose cells walls. The phloem parenchyma are present in pteridophytes and most of the dicotyledonous angiosperms. They are absent in monocots and some of the dicots (Ranunculus). in active phloem, these cells remain active but as soon as the activity of sieve-tubes ceases they become lignified.The main function of phloem parenchma is translocation and storage of food reserves.
(iv) Phloem fibres:
These are also called bast fibers and mostly occur in secondary phloem. The fibers of primary phloem have both cellulose and lignified thickenings. The fibers of secondary phloem are elongated lignified cells with simple pits. The ends of these cells may be pointed needle like or blunt. They are non-living cells that provide mechanical support and give strength and rigidity to the organ.
