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
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
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
Sclerenchyma
Cells also have a support function in
plants but unlike collenchyma cells, they have a hardening agent
and are much more rigid.
Image Credit : http://www.flickr.com/photos/phillipwest/55087615/
