The Plant Tissues
Like other organisms, the cells in a plant are
grouped together into various tissues. These tissues can be
simple, consisting of a single cell type, or complex, consisting
of more than one cell type.
Above and beyond tissues, plants also have a higher level of structure called tissue systems. There are three typical types.
Plant Tissue Systems
Plant tissue systems include:
Dermal Tissue System
The dermal tissue system consists of the epidermis and the periderm. The epidermis is a single layer of closely packed cells. It both covers and protects the plant. It can be thought of as the plant's "skin." Depending on the part of the plant that it covers, the dermal tissue system can be specialized to a certain extent. For instance, the epidermis of a plant's leaves secretes a coating called the cuticle that helps the plant retain water.
The Epidermis is the interface between the plant and its environment. Consequently, many adaptations have evolved to foster existence in terrestrial ecosystems where the leaves are exposed to the atmosphere.
The periderm, also called bark, replaces the epidermis in plants that undergo secondary growth. The periderm consists of cork cells and protects the plant from pathogens, prevents excessive water loss and provides insulation for the plant.
Periderm is a secondary tissue produced by the Cork Cambium or Phellogen. Periderm is a protective tissue. Its cells are called Cork or Phellem. They are dead at maturity but their walls are impregnated with Suberin. Suberin is a waxy material like Cutin & it stains positively with Phloroglucinol & Sudan. Suberin is waterproof and resists microbial degradation. Consequently, cork cells protect the stem or root from excess water loss and the entrance of pathogens. It also acts as an insulator against extreme temperatures. Periderm is commonly called Bark.
Periderm formation is indicated by periclinal
divisions in parenchyma cells.
Vascular Tissue System
Xylem and Phloem throughout the plant make up the vascular tissue system. It allows water and other nutrients to be transported throughout the plant.
Xylem and Phloemare the Vascular Tissues in plants. They are usually found together. The vascular tissues in stems are found within tubular bundles which are continuous over a relatively large distance.
Cells in the xylem have thick lignified walls
which stain red for lignin in most preparations.
Both Xylem and Phloem have cytological traits which distinguish them from the surrounding Ground Tissues. The Lignified walls of the Tracheary Elements in the Xylem are the most distinctive of these. Xylem & Phloem are said to be "complex tissues" because they contain Fibers and Parenchyma as well as Tracheary Elements (Xylem) & Sieve Elements (Phloem).
The most specialized cells of the Xylem are called Tracheary Elements. These serve two functions, physical support & water conduction. Vessel Members (VM) have large openings in their end walls. These are the "Perforation Plates".
The most advanced VM have no end wall obstructions between succeeding Vessel Member. These have "Simple Perforation Plates".
Elongated cells with narrow bores are better for Support.
Wider cells are better for Conduction.
Tracheary Elements also contain many pits on
their side walls.
The ladder-like (scalariform) segments of cell wall which form the Perforation Plate on the end wall of the Vessel Member is a Scalariform Perforation Plate.This cell provides moderate support and fluid conduction.
Tracheids are the other type of Tracheary Element. Tracheids are usually more elongate and narrower than Vessel Members.
However, the most critical difference is that Tracheids are Imperforate. This means that they lack Perforation Plates. Their end walls contain numerous pits, which facilitate the longitudinal transport of water.
Vessel Members are characteristic for
Angiosperms. Tracheids are characteristic for all other vascular
plants with a few exceptions. The most advanced angiosperms have
both types of tracheary elements in their xylem.Traced endwalls
only contain Pits.
Comparison of Tracheids and Vessel Members : A Vessel is composed of several Vessel Members. There is no collective term for a longitudinal series of Tracheids. The disparity in radial and longitudinal sizes demonstrate that Tracheids can supply more structural support than Vessels & that Vessels can conduct more water.
Ground Tissue System
The ground tissue system synthesizes organic compounds, supports the plant and provides storage for the plant. It is mostly made up of parenchyma cells but can also include some collenchyma and sclerenchyma cells as well.
Parenchyma cells are usually depicted as the "typical" plant cell because they are not very specialized. These cells synthesize and store organic products in the plant. Most of the plant's metabolism takes place in these cells.
Plant Parenchyma Cell
Collenchyma Cells have a support function in plants, particularly in young plants. These cells help to support plants while not restraining growth due to their lack of secondary walls and the absence of a hardening agent in their primary walls.
Sclerenchyma Cells also have a support function in plants but unlike collenchyma cells, they have a hardening agent and are much more rigid.
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