Created by: Swati Sinha 1 year, 7 months ago
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Simple Permanent Tissue – Sclerenchyma
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Complex Permanent Tissue - Xylem

Complex permanent tissue consists of a number of dissimilar cells which perform a common function. It forms vascular or conducting tissue in the plant. They are of two types: Xylem & Phloem.

Xylem is responsible for conduction of mineral salt and water from roots to upper region and also provides mechanical support to the plant body. Xylem is also called as wood and forms the bulk of roots and stem of vascular plants. Xylem is composed of three types of cells some of which are living and some non-living.

Tracheary Elements: These participate in conduction of sap (water and minerals). They are divided into two types – tracheids and vessels.

Vessels: Vessels are elongated tubular structures with a wide lumen. It is formed by a series of cylindrical cells which are joined end-to-end and have perforated walls. The perforated end walls are called perforation plates. Those with a single large opening are called simple perforation plates whereas the ones with several openings are called as multiple perforation plates. The vessels are dead, thick walled and lignified. On the basis of how the deposition of lignin occurs on vessel wall, the vessels are called as annular, spiral, reticular, sclariform and pitted(most commonly found). Vessels are present in the xylem of angiosperm and are absent in gymnosperm and pteridophyte.

Tracheids: Tracheids are elongated, thick walled, dead tubular cells with lignified walls. Based on the lignification, tracheids are also classified as annular, spiral, reticulate, sclariform and pitted. The tracheids are smaller than vessels and form chief conducting element of xylem in gymnosperm and pteridophyte where vessels are absent. Only few tracheids are present in angiosperm in association with vessels.

Wood Fibre/ Xylem Fibres: They are sclerenchyma fibres. They are distinguished from tracheids on basis of their comparatively thicker walls and obliterated central lumens. They are mostly abundant in secondary xylem and their main function is to provide mechanical strength to the xylem and the entire plant body. They also help vessel and tracheids in conduction of sap. These fibres are of two types: libriform fibres have thick cell walls, simple pits and obliterated central lumens; fibre tracheids are intermediary between fibres and tracheids with thin cell walls. 

Wood Parenchyma: are parenchymatous cells present in the xylem. It is the only living component of the xylem. The cells are thin or thick walled with simple pits. These cells store food material in the form of starch or fat or such other materials as tannin. They assist vessels and tracheids in the lateral conduction of sap.

Xylem can be distinguished into two types on basis of the time of origin and internal structure – protoxylem and metaxylem.

Protoxylem as the name suggests is the first or easily formed xylem. It consists of small tracheids and vessels with annular or spiral lignification. The thickening of cell walls starts before the xylem cells are completely elongated.

Metaxylem is formed at a later stage. It consists of large tracheids and vessels and the lignification occurs in reticulate, sclariform and pitted manner. Only after the completion of elongation of xylem cells, the thickening of cell wall occurs. 

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Complex Permanent Tissue: Phloem
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