plant stem with several layers of different tissue types: • In
plant anatomy, tissues are categorized broadly into three tissue systems: the
epidermis, the
ground tissue, and the
vascular tissue. •
Epidermis – Cells forming the outer surface of the
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
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 Meristematic tissue consists of actively dividing cells and leads to an 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 walls. 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
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 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
dicotyledons 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 intercellular spaces between them. Each cell contains a dense
cytoplasm and a prominent
cell nucleus. The dense
protoplasm of meristematic cells contains very few
vacuoles. Normally the meristematic cells are oval,
polygonal, 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 that 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
Idioblast 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 Collenchyma (Greek,
Colla means gum;
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 (
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 Sclerenchyma (Greek,
Sclerous means hard;
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 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: •
Tracheids •
Vessels (or tracheae) • Xylem fibers or Xylem sclerenchyma • Xylem parenchyma '', 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 cells are vessels, 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. 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 is also 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, which 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. ==Animal tissue==