Complex Tissues
A group of dissimilar cells which functions as an unit and perform a specific function are called as complex tissues. In plant two types of complex tissues are found – xylem and phloem.They are distributed in plant organs in the form of vascular bundles. They help in the conduction of water, mineral salts and organic substances. They are also referred to as vascular tissues.
They are absent in thallophytes and bryophytes. Vascular tissues first appeared in pteridophytes and gradually developed into complex structures in gymnosperms and angiosperms. The plant possessing vascular tissues are included in Tracheophyta.
Xylem (xylos =wood):
Complex tissue that conducts water and mineral salts from root to the aerial parts of the plant body is called as xylem. The term xylem was coined by K. Nageli and is derived from the Greek work Xylos, meaning wood. Xylem constitutes the most important part of a vascular bundle is also known as hydrome.
The xylem is composed of following types of elements or cells:
1. Tracheary Elements 2. Xylem Fibres and 3. Xylem Parenchyma.
Among these, except xylem parenchyma, all other elements are lignified, thick walled and dead.
Tracheary Elements:
They are specialized dead cells with thick and lignified walls which conducts water and mineral salts and which also, to certain degree, have a supporting function. The tracheary elements are highly specialized cells and are of two types – Tracheids and Vessels
Tracheids (Latin trachea – windpipe)
The term tracheid was introduced by Sanio in 1863. The tracheid is a fundamental cell type in xylem. They are elongate cells, with broad middle part and tapering ends with hard, thick and lignified walls. They are dead empty cells without any protoplasm.
The cell cavity or lumen of the tracheid is large. The walls of tracheids are provided with abundant bordered pits. In transverse section, the tracheid may appear angular, circular or polyhedral in shape. The tracheids possess varioius kinds of thickenings like annular, spiral, scalariform, reticulate or pitted.
In vascular bundles, the tracheids are arranged in parallel rows with partial overlapping and are intertwined to form continuous conducting strands. The continuity between the overlapping tracheids is maintained through the thin pit pairs on their common walls. The conduction of water and salts takes place through these pits. The perforations or opening at the end of tracheids cells are absent.
The tracheids are specially adapted to function of conduction. The thick and rigid walls of tracheids also help in mechanical support.
In pteridophytes and gymnosperms tracheids are the main elements of water conduction. Tracheids alone occur in pteridophytes and gymnosperms. Whereas in angiosperms they occur associated with the vessels. The tracheids are primitive to vessels.
Vessels Elements:
Vessel elements are elongated, wide, cylindrical cells with thick lignified secondary walls and are without protoplast. They are arranged end to end and are joined into long continuous tubes called vessels.
The end walls of the vessel elements are perforated. The wall area bearing perforations is called the perforation plate. The perforation plate when bears single opening or pore is called as simple perforation. If there are two or more openings, they are known as multiple perforations.
As vessel elements are arranged end to end and sap moves freely through the perforation areas, vessels are more efficient in conduction of sap than the tracheids.
Phylogenetically vessels are more advanced than tracheids and are evolved from them. They are the chief elements of conduction in angiosperms and help in the transportation of water and mineral salts. However, their present in some pteridophytes like Equisetum and Selaginella. They are also found in some gymnosperms like Ephedra, Welwitschia, Gnetum. Although vessels are characteristic feature of angiosperms, they are absent in some angiosperms like Trochodentron, Drimys (vesselless angiosperms).In monocotyledons like Yucca, Dracaena they are absent from the stems and leaves. In aquatic plants, parasities and succulent plants vessels are less developed.
Secondary Thickenings on the Tracheary Elements:
The secondary walls of tracheary elements both tracheids and vessels show wide variety of thickenings. The thickening in them is due to the deposition of lignin in the interior of the cell wall. The deposition of lignin takes the following patterns:
1. Annular – Circular or ring like secondary thickenings
2. Spiral or Helical – Spring or spiral like secondary thickenings
3. Scalariform – Ladder like secondary thickenings
4. Reticulate – Net like secondary thickeings
5. Pitted – the lignin deposition on the wall is uniform but interrupted by pits.
In many plants the secondary wall thickening of the first formed xylem (protoxylem) is annular or helical. In later formed tracheary elements (early formed metaxylem) the thickening is scalariform. In tracheary elements formed at a still later stage, the wall thickening is in the form of a network i.e., reticulate thickening. In the most advanced elements, the secondary wall is interrupted only at its pits, such elements elements are termed pitted elements.
Xylem Parenchyma:
The parenchyma cells frequently found within the xylem tissue are referred to as xylem or wood parenchyma. These cells are more or less elongated, placed end to end and may be thick or thin walled.
Xylem parenchyma is the only living component of xylem. Parenchyma cells remain alive as long as xylem tissue is active and performs its function as conductive tissue. Parenchyma cells are present in primary and secondary xylem. They form a major part of primary xylem. In secondary xylem the xylem parenchyma is of two types –
(a) - Axial parenchyma : parenchyma cells arranged end to end in vertical rows among the tracheary elements. These cells are rectangular to elongated with horizontal end walls,
(b) – Radial or Ray parenchyma : allinged horizontally among the tracheary elements.
Much taxonomic importance is attached to the type of distribution of parenchyma. When xylem parenchyma is associated with vessel, then it is called as paratracheal parenchyma. Freely distributed parenchyma which is not associated with vessels is known as apotracheal parenchyma.
Functions:
Storage of food materials such as starch, oils and fats and other materials like tannins, crystals.
Parenchyma cells are always involved directly or indirectly in translocation of water and mineral salts.
Ray parenchyma in secondary xylem is concerned with radial conduction.
Xylem Fibres or Wood Fibres:
The long, slender, pointed, dead sclerenchymatous cells present in both primary and secondary xylem are called as xylem fibres or wood fibres. They develop from the same meristem which gives rise to other xylem elements.
They have thick, lignified walls with narrow lumen. On the basis of wall thickness and nature of pits, two main types of xylem fibres are distinguishable:
a. Libriform fibres : they are extremely thick walled with very narrow lumen. Pits are very small and simple pits and obliquely lenticular. Because of their similarity to phloem fibres they are named as libriform fibres (liber = phloem fibres).
b. Fibre Tracheids : they are shorter than libriform fibres and shows moderately thickened walls and fairly large lumen. They have bordered pits. They are supposed to have developed from non-functional tracheids by reduction.
In secondary xylem of dicots another type of fibre is identified known as Gelatinous or mucilaginous fibre. The innermost layer of the secondary wall in this fibre contain a lot of α – cellulose and is poor in lignin. This layer absorbs much water and may swell to fill up the entire lumen of the fibre.
Functions:
Provide mechanical strength. According to recent studies fibres retain their living conditions for a long time and are concerned with storage of reserve materials like starch and oils.
Commercially, fibres are extracted and used in textile and fibre industries.
Formation of Xylem:
Primary xylem : Xylem developed from procambium in the primary body is called as primary xylem.
The first formed primary xylem is called as protoxylem and latter formed primary xylem is called metaxylem. The protoxylem elements are smaller and have annular and spiral thickenings. The metaxylem elements are larger and have scalariform, reticulate and pitted thickenings.
Depending on the arrangement of protoxylem in relation to metaxylem in a vascular bundle, primary xylem is of 3 types:
i. Exarch Xylem : if protoxylem is present towards outside facing epidermis and metaxylem toward inside, the xylem is called as exarch xylem. E.g., Roots of dicots and monocots.
ii. Endarch Xylem: if protoxylem is present towards inside and metaxylem towards outside facing the epidermis, the xylem is called endarch. E.g., Stems of dicots and monocots.
iii. Mesarch Xylem: if protoxylem is in the centre occupied on either side by metaxylem, it is called mesarch xylem. E.g., Ferns.
Secondary Xylem: xylem formed by the activity of vascular cambium during secondary growth is called secondary xylem or wood. It is abundant in dicot stems, roots after secondary growth.
Comparision of Tracheid and Vessel
Tracheids. Vessels
Short, usually upto 1mm in length. Comparatively longer and may
reach upto 10cm in length.
Consists of single elongated cell consists of row of cells placed
with tapering end walls. one above the other
The tracheids are placed one above The vessels are tubular and
the other are separated by cross have no cross walls.
walls which bear bordered pits.
They end walls are. The end walls are perforated
not perforated.
Present in almost all vascular plants. Absent in pteridophytes and
They are chief elements of conduction gymnosperms. They are
in pteridophytes and gymnosperms. chief elements of conduction in angiosperm
a. Libriform fibres : they are extremely thick walled with very narrow lumen. Pits are very small and simple pits and obliquely lenticular. Because of their similarity to phloem fibres they are named as libriform fibres (liber = phloem fibres).
b. Fibre Tracheids : they are shorter than libriform fibres and shows moderately thickened walls and fairly large lumen. They have bordered pits. They are supposed to have developed from non-functional tracheids by reduction.
In secondary xylem of dicots another type of fibre is identified known as Gelatinous or mucilaginous fibre. The innermost layer of the secondary wall in this fibre contain a lot of α – cellulose and is poor in lignin. This layer absorbs much water and may swell to fill up the entire lumen of the fibre.
Functions:
Provide mechanical strength. According to recent studies fibres retain their living conditions for a long time and are concerned with storage of reserve materials like starch and oils.
Commercially, fibres are extracted and used in textile and fibre industries.
Formation of Xylem:
Primary xylem : Xylem developed from procambium in the primary body is called as primary xylem.
The first formed primary xylem is called as protoxylem and latter formed primary xylem is called metaxylem. The protoxylem elements are smaller and have annular and spiral thickenings. The metaxylem elements are larger and have scalariform, reticulate and pitted thickenings.
Depending on the arrangement of protoxylem in relation to metaxylem in a vascular bundle, primary xylem is of 3 types:
i. Exarch Xylem : if protoxylem is present towards outside facing epidermis and metaxylem toward inside, the xylem is called as exarch xylem. E.g., Roots of dicots and monocots.
ii. Endarch Xylem: if protoxylem is present towards inside and metaxylem towards outside facing the epidermis, the xylem is called endarch. E.g., Stems of dicots and monocots.
iii. Mesarch Xylem: if protoxylem is in the centre occupied on either side by metaxylem, it is called mesarch xylem. E.g., Ferns.
Secondary Xylem: xylem formed by the activity of vascular cambium during secondary growth is called secondary xylem or wood. It is abundant in dicot stems, roots after secondary growth.
Comparision of Tracheid and Vessel
Tracheids. Vessels
Short, usually upto 1mm in length. Comparatively longer and may
reach upto 10cm in length.
with tapering end walls. one above the other
The tracheids are placed one above The vessels are tubular and
the other are separated by cross have no cross walls.
walls which bear bordered pits.
They end walls are. The end walls are perforated
not perforated.
Present in almost all vascular plants. Absent in pteridophytes and
They are chief elements of conduction gymnosperms. They are
in pteridophytes and gymnosperms. chief elements of conduction in angiosperm
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