Editing Tissue (biology) (section)

== Plant tissue ==

[[File:Stem-histology-cross-section-tag.svg|thumb|right|Cross-section of a [[flax]] plant stem with several layers of different tissue types: {{olist
    |[[Pith]]
    |[[Xylem#Protoxylem and metaxylem|Protoxylem]]
    |[[Xylem]]
    |[[Phloem]]
    |[[Sclerenchyma]] ([[bast fiber]])
    |[[cortex (botany)|Cortex]]
    |[[epidermis (botany)|Epidermis]]
}}]]

In [[plant anatomy]], tissues are categorized broadly into three tissue systems: the [[epidermis (botany)|epidermis]], the [[ground tissue]], and the [[vascular tissue]].
* '''Epidermis''' – Cells forming the outer surface of the [[leaf|leaves]] and of the young plant body.
* '''Vascular tissue''' – The primary components of vascular tissue are the [[xylem]] and [[phloem]]. These transport fluids and nutrients internally.
* '''Ground tissue''' – Ground tissue is less [[cellular differentiation|differentiated]] than other tissues. Ground tissue manufactures nutrients by [[photosynthesis]] and stores reserve nutrients.

Plant tissues can also be divided differently into two types:

# Meristematic tissues
# Permanent tissues.

=== Meristematic tissue ===
{{See also|Plant stem cell}}

[[Meristem|Meristematic tissue]] consists of actively dividing cells and leads to increase in length and thickness of the plant. The primary growth of a plant occurs only in certain specific regions, such as in the tips of stems or roots. It is in these regions that meristematic tissue is present. Cells of this type of tissue are roughly spherical or polyhedral to rectangular in shape, with thin [[cell wall]]s. New cells produced by meristem are initially those of meristem itself, but as the new cells grow and mature, their characteristics slowly change and they become differentiated as components of meristematic tissue, being classified as:

1.Primary meristem.
* '''Apical meristem ''': Present at the growing tips of stems and roots, they increase the length of the stem and root. They form growing parts at the [[wikt:apex|apices]] of roots and stems and are responsible for the increase in length, also called primary growth. This meristem is responsible for the linear growth of an organ.
2.Secondary meristem.

* ''' Lateral meristem''': Cells which mainly divide in one [[Plane (geometry)|plane]] and cause the organ to increase in diameter and girth. Lateral meristem usually occurs beneath the bark of the tree as [[cork cambium]] and in vascular bundles of [[dicotyledon]]s as [[vascular cambium]]. The activity of this cambium forms secondary growth.
* '''Intercalary meristem''': Located between permanent tissues, it is usually present at the base of the node, internode, and on leaf base. They are responsible for growth in length of the plant and increasing the size of the internode. They result in branch formation and growth.

The cells of [[meristematic tissue]] are similar in structure and have a thin and elastic primary cell wall made of [[cellulose]]. They are compactly arranged without inter-cellular spaces between them. Each cell contains a dense [[cytoplasm]] and a prominent [[cell nucleus]]. The dense [[protoplasm]] of meristematic cells contains very few [[vacuole]]s. Normally the meristematic cells are oval, [[polygon]]al, or [[rectangular]] in shape.

Meristematic tissue cells have a large nucleus with small or no vacuoles because they have no need to store anything. Their basic function is to multiply and increase the girth and length of the plant, with no intercellular spaces.

=== Permanent tissues ===
Permanent tissues may be defined as a group of living or dead cells formed by meristematic tissue and have lost their ability to divide and have permanently placed at fixed positions in the plant body. Meristematic tissues that take up a specific role lose the ability to divide. This process of taking up a permanent shape, size and a function is called [[cellular differentiation]]. Cells of meristematic tissue differentiate to form different types of permanent tissues. There are 2 types of permanent tissues:
# simple permanent tissues
# complex permanent tissues

==== Simple permanent tissue ====
Simple permanent tissue is a group of cells which are similar in origin, structure, and function. They are of three types:
# [[Parenchyma]]
# [[Collenchyma]]
# [[Sclerenchyma]]

===== Parenchyma =====
[[Parenchyma]] (Greek, ''para'' – 'beside'; enchyma– ''infusion'' – 'tissue') is the bulk of a substance. In plants, it consists of relatively unspecialized living cells with thin cell walls that are usually loosely packed so that intercellular spaces are found between cells of this tissue. These are generally isodiametric, in shape. They contain small number of vacuoles or sometimes they even may not contain any vacuole. Even if they do so the vacuole  is of much smaller size than of normal animal cells. This tissue provides support to plants and also stores food. [[Chlorenchyma]] is a special type of parenchyma that contains [[chlorophyll]] and performs photosynthesis. In aquatic plants, [[aerenchyma]] tissues, or large air cavities, give support to float on water by making them buoyant. Parenchyma cells called idioblasts have metabolic waste. Spindle shaped fibers are also present in this cell to support them and known as prosenchyma, succulent parenchyma also noted. In [[xerophytes]], parenchyma tissues store water.

=====Collenchyma=====
[[File:Plant cell type collenchyma.png|thumb|Cross section of collenchyma cells]]

[[Collenchyma]] (Greek, 'Colla' means gum and 'enchyma' means infusion) is a living tissue of primary body like [[Parenchyma]]. Cells are thin-walled but possess thickening of [[cellulose]], water and [[pectin]] substances ([[Hydroxypropyl cellulose|pectocellulose]]) at the corners where a number of cells join. This tissue gives tensile strength to the plant and the cells are compactly arranged and have very little inter-cellular spaces. It occurs chiefly in [[hypodermis]] of stems and leaves. It is absent in [[monocots]] and in roots.

Collenchymatous tissue acts as a supporting tissue in stems of young plants. It provides mechanical support, elasticity, and tensile strength to the plant body. It helps in manufacturing sugar and storing it as starch. It is present in the margin of leaves and resists tearing effect of the wind.

===== Sclerenchyma =====
[[Ground tissue#Sclerenchyma|Sclerenchyma]] (Greek, ''Sclerous'' means hard and ''enchyma'' means infusion) consists of thick-walled, dead cells and protoplasm is negligible. These cells have hard and extremely thick secondary walls due to uniform distribution and high secretion of [[lignin]] and have a function of providing mechanical support. They do not have inter-cellular spaces between them. Lignin deposition is so thick that the cell walls become stronger, rigid and impermeable to water, which are also known as a stone cells or sclereids. These tissues are mainly of two types: sclerenchyma fiber and sclereids.
Sclerenchyma fiber cells have a narrow lumen and are long, narrow and unicellular. Fibers are elongated cells that are strong and flexible, often used in ropes. Sclereids have extremely thick cell walls and are brittle, and are found in nutshells and legumes.

===== Epidermis =====
The entire surface of the plant consists of a single layer of cells called epidermis or surface tissue. The entire surface of the plant has this outer layer of the epidermis. Hence it is also called  surface tissue. Most of the epidermal cells are relatively flat. The outer and lateral walls of the cell are often thicker than the inner walls. The cells form a continuous sheet without intercellular spaces. It protects all parts of the plant. The outer epidermis is coated with a waxy thick layer called cutin which prevents loss of water. The epidermis also consists of [[stomata]] (singular:stoma) which helps in [[transpiration]].

==== Complex permanent tissue ====
The complex permanent tissue consists of more than one type of cells having a common origin which work together as a unit. Complex tissues are mainly concerned with the transportation of mineral nutrients, organic solutes (food materials), and water. That's why it is also known as conducting and vascular tissue. The common types of complex permanent tissue are:
* [[Xylem]] (or wood)
* [[Phloem]] (or bast).

Xylem and phloem together form vascular bundles.

===== Xylem =====
{{Anchor|Ray cell}}
[[Xylem]] (Greek, ''xylos'' = wood) serves as a chief conducting tissue of vascular plants. It is responsible for the conduction of water and inorganic solutes. Xylem consists of four kinds of cells:
* [[Tracheid]]s
* [[Vessel element|Vessels]] (or tracheae)
* Xylem fibers or Xylem sclerenchyma
* Xylem parenchyma

[[File:Xylem rays (Tilia Americana).jpg|thumb|right|Cross section of 2-year-old ''[[Tilia americana]]'', highlighting xylem ray shape and orientation]]

Xylem tissue is organised in a tube-like fashion along the main axes of stems and roots. It consists of a combination of parenchyma cells, fibers, vessels, tracheids, and ray cells. Longer tubes made up of individual cellssels tracheids, while vessel members are open at each end. Internally, there may be bars of wall material extending across the open space. These cells are joined end to end to form long tubes. Vessel members and tracheids are dead at maturity. Tracheids have thick secondary cell walls and are tapered at the ends. They do not have end openings such as the vessels. The  end overlap with each other, with pairs of pits present. The pit pairs allow water to pass from cell to cell.

Though most conduction in xylem tissue is vertical, lateral conduction along the diameter of a stem is facilitated via rays.{{cn|date=August 2022}} Rays are horizontal rows of long-living parenchyma cells that arise out of the vascular cambium.

===== Phloem =====
[[Phloem]] consists of:
* [[Sieve tube]]
* [[Companion cell]]
* Phloem fiber
* Phloem parenchyma.

Phloem is an equally important plant tissue as it also is part of the 'plumbing system' of a plant. Primarily, phloem carries dissolved food substances throughout the plant. This conduction system is composed of sieve-tube member and companion cells, that are without secondary walls. The parent cells of the vascular cambium produce both xylem and phloem. This usually also includes fibers, parenchyma and ray cells. Sieve tubes are formed from sieve-tube members laid end to end. The end walls, unlike vessel members in xylem, do not have openings. The end walls, however, are full of small pores where cytoplasm extends from cell to cell. These porous connections are called sieve plates. In spite of the fact that their cytoplasm is actively involved in the conduction of food materials, sieve-tube members do not have nuclei at maturity. It is the companion cells that are nestled between sieve-tube members that function in some manner bringing about the conduction of food. Sieve-tube members that are alive contain a polymer called callose, a carbohydrate polymer, forming the callus pad/callus, the colourless substance that covers the sieve plate. Callose stays in solution as long as the cell contents are under pressure. Phloem transports food and materials in plants upwards and downwards as required.