Complex permanent tissues are heterogenous tissues i.e., they are made up of different types of cells, all of which co-ordinate to perform a particular function. These tissues are meant for conduction of water and food materials. So they are called vascular tissues or conducting tissues.
They are present in all plants belonging to pteridophytes, gymnosperms and angiosperms. Therefore these plants are together known as vascular plants or tracheophytes. The primary xylem and phloem are differented from the procambium.
Complex permanent tissues are of two types – Xylem and Phloem.
Type # 1. Xylem:
Xylem is a complex permanent tissue which Conducts water and mineral nutrients upwards from the root to the leaves. It also gives mechanical strength to the organ in which it is present.
The xylem is composed of four different elements:
(i) Tracheids.
(ii) Vessels or tracheae.
(iii) Xylem parenchyma.
(iv) Xylem fibres.
Of these, the tracheids and the trachea are described as essential elements since they are directly involved in the translocation of water and mineral salts. Xylem parenchyma and xylem fibres are described as associated elements, since they are only supporting structures. The tracheids, trachea and xylem fibres constitute non-living components, while xylem parenchyma represents the only living component of the tissue.
Tracheary Elements:
Two fundamental types of tracheary elements occur in the xylem-tracheids and vessel members (vessel elements). They are the main water conducting elements of the xylem. They show close resemblance to insect trachea. Hence they are called tracheary elements.
(i) Tracheids:
Tracheids are elongated tubular primitive cells with tapering end walls. They are dead cells and they do not contain protoplasts. Secondary walls in these cells are lignified. Tracheids are arranged parallel to the long axis of the plant body. They have an empty lumen through which water passes without any obstruction.
Depending upon the deposition of lignin on the primary cell wall, tracheids may be classified as annular, spiral, scalariform or pitted. Tracheids are the only water conducting elements in ferns and gymnosperms. Tracheids have pit pairs on their common walls with other tracheids. Water flows from one tracheid to another through pits in the cell walls.
(ii) Vessels or Tracheae:
Vessels are the continuous pipe like structures of the xylem formed by joining several vessel members (vessel elements) end to end. Vessel members are dead, lignified cylindrical cells shorter than the tracheids. Their end walls are absent. Vessels are absent in gymnosperms. Vessels are found in the xylem of angiosperms.
Like tracheids, the walls of the vessels are also thickened in various ways.
According to the mode of thickening of the secondary wall vessels are of various types:
a. Annular Vessels:
The secondary thickenings are in the form of rings placed more or less at an equal distance.
b. Spiral Vessels:
The secondary thickenings are in the form of a coil or a spring or a helix. Scalariform Vessels the secondary thickenings appear in the form of close bands resembling the steps of a ladder.
c. Reticulate Vessels:
The secondary thickenings appear in the form of a network.
d. Pitted Vessels:
Lignin deposition is almost uniform over the wall, except for small pores here and there. These pores are called pits. They are two types- simple pits and bordered pits.
Pits are cavities in the secondary wall. In a pit region, the primary wall is not covered by secondary layers. Such a pit is a simple pit. In a bordered pit the secondary wall arches over the pit cavity. Fundamentally, a pit consists of a pit cavity and a pit membrane. The pit cavity is open internally to the lumen of the cell and is closed by the pit membrane along the line of junction of two cells.
(iii) Xylem Parenchyma:
Parenchyma cells associated with the xylem form xylem parenchyma. These cells form the only living component of the xylem. Xylem parenchyma stores food in the form of starch. These cells assist directly or indirectly in the conduction of water upward through the vessels and tracheids.
(iv) Xylem Fibres:
Sclerenchyma fibres associated with the xylem are called xylem fibres. These fibres have lignified cell walls. They provide mechanical strength to the plant body.
Types of Xylem:
1. Primary Xylem:
The xylem differentiating in the primary plant body is the primary xylem. The immediate precursor of this xylem is the procambium.
2. Secondary Xylem:
In a plant after completion of primary growth, it forms secondary tissues by the activity of vascular cambium. The xylem produced by the vascular cambium is called secondary xylem. It is also called wood.
When the primary xylem is studied, some developmental and structural differences are noticed in the earlier and later formed parts of this tissue. Protoxylem is the tissue which appears at the beginning of vascular differentiation. It is the first formed xylem. In the stem and leaf, the protoxylem matures before these organs undergo extensive elongation. The tracheids and vessels of the protoxylem commonly have annular and spiral thickenings.
The xylem that appears after the protoxylem, when the elongation of the stem and leaf is completed is called metaxylem. The tracheids and vessels of the metaxylem have scalariform, reticulate and pitted thickenings. The metaxylem tracheary elements are wider than protoxylem elements. The xylem shows three fundamental patterns of differentiation.
When the development of the xylem takes place towards the centre of the axis, or if the protoxylem develops towards the periphery, the xylem is said to be exarch. E.g. – roots of vascular plants. If the development of the xylem is towards the periphery i.e., away from the axis or the protoxylem elements develop towards the centre, the xylem is said to be endarch. E.g. – stems of seed plants.
In the third case, the differentiation progresses in two or more directions from the first mature xylem elements. This type of primary xylem is called mesarch. E.g. – rhizome of ferns.
Functions of the Xylem:
I. Xylem is the conducting tissue of vascular plants taking part in the upward conduction of water and solutes. Tracheids and trachea are the actual conducting elements of the xylem.
II. Tracheids, trachea and xylem fibres provide mechanical support to plants.
III. Xylem parenchyma stores water and food.
Type # 2. Phloem: (Gr., phloos = bark):
This is the food conducting complex permanent tissue. The term phloem was coined by Nageli in 1958.
It is composed of:
i. Sieve elements.
ii. Companion cells.
iii. Phloem fibres.
iv. Phloem parenchyma.
Of these, the sieve elements and companion cells are directly involved in the translocation of the organic substances. Hence, they are called as essential elements. Phloem parenchyma and phloem fibres are described as associated elements, since they play only a supporting role in the process. The sieve elements, the companion cells and the phloem parenchyma are the living components of the phloem, while the phloem fibres are the only non-living component of the phloem.
i. Sieve Elements:
The conducting elements of the phloem are collectively known as sieve elements. They are segregated into less specialized sieve cells and more specialised sieve tube members or sieve tube elements.
a. Sieve Cells:
Sieve cells have unspecialized sieve areas. These sieve areas are depressed thin areas on the lateral wall of the sieve cell. They have numerous pores through which the adjacent sieve elements are interconnected by connecting strands. A defined sieve plate is absent. Sieve cells are long and slender, cylindrical cells with tapering end. They are found in the phloem of pteridophytes and gymnosperms.
Sieve Tube Members:
Sieve tube members are specialized cells with the end walls bearing a clear defined sieve plate. Sieve tube members are placed end to end in a long series forming a sieve tube.
The most characteristic feature of the sieve tube is the presence of thickened cross walls between two adjacent sieve tube members. These are known as sieve plates. Each sieve plate has a large number of perforations. The opening of the sieve plate is delimited by tubular thickenings of callose (carbohydrate), in winter, callose completely blocks the pores of the sieve plates, but in spring, when the active season begins, it gets dissolved. In old sieve tubes callose forms a permanent deposit.
Sieve tubes are present in angiosperms. The sieve tube members in a sieve tube have a protoplast which is enucleate at maturity. It is the only living plant cell without a nucleus. Each sieve tube member is associated with a companion cell. These two cells are sister cells as they arise by the mitotic division of a common mother cell.
In mature sieve tubes, cytoplasm is a thin layer close to the cell wall. In many dicotyledons, the sieve tube members have variable amounts of a viscous proteinaceous substance called slime. The slime originates in the cytoplasm in the form of discrete bodies called as slime bodies.
In young sieve elements, these bodies consist of aggregates of tubules, designated as P-protein (phloem-protein). Sieve tube members and companion cells are connected by plasmodesmata. The cytoplasm of the sieve tube member lacks a vacuole, tonoplast, Golgi and ribosome.
Function:
Sieve tubes are used for the longitudinal transmission of prepared food materials from the leaves to the storage organs in the downward direction and later from the storage organs to the growing regions in the upward direction.
ii. Companion Cells:
These are living nucleated cells associated with each sieve tube member. They are specialised parenchyma cells. Usually a single companion cell is found associated with a sieve tube member. The cytoplasm of the sieve tube element and companion cells are connected by thin cytoplasmic strands called plasmodesmata, passing through the pit membranes in their side walls. Companion cells are absent in the phloem of pteridophytes and gymnosperms. Instead of these cells, they have albuminous cells.
Function:
Companion cells along with phloem parenchyma play an important role in the maintenance of a pressure gradient in the sieve tubes. They form a link between sieve tube cells and other cells and control the passage of materials.
iii. Phloem Parenchyma:
These are parenchyma cells found associated with the phloem. They are concerned with many activities such as storage of starch and other organic food materials. Phloem of parenchyma is absent in the phloem of most monocots and a few dicots.
iv. Phloem Fibres:
They are sclerenchyma fibres found associated with the secondary phloem. They are lignified cells and provide mechanical support to the plant body. Phloem fibres are economically important. E.g. – flax, hemp and jute.
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