The Tissues

• It’s a group of cells of common origin and regularly carrying out common functions. Different kinds of tissues constitute a plant.

Tissues are bifurcated into two main groups (based on whether the cells formed are capable of subsequent division or not), namely,

· Meristematic tissues

· Permanent tissues

Meristematic Tissues:

• The growth of plants is concentrated mainly to specific regions of active cell division called meristems and there exists different kinds of meristems in plants. The meristems occurrence at the tips of roots and shoots, that yield primary tissues are called as apical meristems.

Apical meristem: (a) Root (b) Shoot

• Root apical meristem lodges the tip of a root while the shoot apical meristem inhabits the distant most region of the stem axis. 

• In the course of the formation of leaves and elongation of stem, some cells ‘left behind’ from shoot apical meristem, establish the axillary bud. These buds present in the axils of leaves and are accomplished of forming a branch or a flower.

• The meristem occurrence between mature tissues is termed as intercalary meristem. They occur in grasses and renew parts taken away by the eating herbivores. 

• Both apical and intercalary meristems are primary meristems, due their appearance early in their life of a plant and contribute to the creation of the primary plant body.

• The meristem that occurs in the mature regions of roots and shoots of many plants, predominantly those produce woody axis and appear later than primary meristem is called the secondary or lateral meristem.

• Fascicular vascular cambium, inter fascicular cambium and cork-cambium are examples of lateral meristems. These are responsible for producing the secondary tissues and are cylindrical meristems.

• Following divisions of cells in both primary and as well as secondary meristems, the newly formed cells become operationally and functionally specialised and lose their capacity to divide. Such cells are termed permanent or mature cells and constitute the permanent tissues.

• During the formation of the primary plant body, precise areas of the apical meristem produce dermal tissues, ground tissues and vascular tissues.

Permanent Tissues:

• The cells of the permanent tissues do not usually split more.

• Permanent tissues having similar structure and function in all cells are called simple tissues.

• Permanent tissues having various dissimilar types of cells are called complex tissues.

Simple Tissues:

• A simple tissue is of only one type of cell. 

The several simple tissues in plants are

· Parenchyma

· Collenchyma

· Sclerenchyma

Parenchyma:

· Parenchyma forms the major constituent within organs and their cells of the parenchyma are generally isodiametric.

· They are spherical, oval, round, polygonal or elongated in shape, their walls are thin, made up of cellulose and are either closely packed or have small intercellular spaces.

· The parenchyma performs various functions like photosynthesis, storage, secretion.

Simple tissues: Parenchyma

Collenchyma:

· The collenchyma occurs in layers below the epidermis in dicotyledonous plants.

· It is found either as a homogeneous layer or in patches, consists of cells which are much thickened at the corners due to a deposition of cellulose, hemicellulose and pectin.

· Collenchymatous cells may be oval, spherical or polygonal, often contain chloroplasts, these cells assimilate food when they contain chloroplasts and intercellular spaces are absent.

· They provide mechanical support to the growing parts of the plant such as young stem and petiole of a leaf.

Simple tissues: Collenchymatous

Sclerenchyma:

· Sclerenchyma consists of long, narrow cells with thick and lignified cell walls having a few or numerous pits and are usually dead, without protoplasts.

· On the basis of variation in form, structure, origin and development, sclerenchyma may be either fibres or sclereids.

· The fibres are thick-walled, elongated and pointed cells, generally occurring in groups, in various parts of the plant.

· The sclereids are spherical, oval or cylindrical, highly thickened dead cells with very narrow cavities (lumen), commonly found in the fruit walls of nuts; pulp of fruits like guava, pear and sapota; seed coats of legumes and leaves of tea. Sclerenchyma provides mechanical support to organs.

Simple tissues: Sclerenchyma

Complex Tissues:

• The complex tissues are made of more than one type of cells and these work together as a unit. 

• Xylem and phloem constitute the complex tissues in plants. 

Xylem:

Xylem

• Xylem functions as a conducting tissue for water and minerals from roots to the stem and leaves and this also affords mechanical strength to the plant parts.

It is composed of four different kinds of elements, namely,

· Tracheids

· Vessels

· Xylem fibres

· Xylem parenchyma

• Gymnosperms lack vessels in their xylem.

Tracheids: Tracheids are elongated or tube like cells with thick and lignified walls, narrowing ends and are dead without protoplasm. The inner layers of the cell walls have thickenings with varied forms. In flowering plants, tracheids and vessels are the main source of water transporting elements.

Vessels: Vessel is a long cylindrical tube-like structure made up of many cells called vessel members, each with lignified walls and a large central cavity and the vessel cells are also devoid of protoplasm. Vessel members are interconnected through perforations in their common walls and the presence of vessels is a characteristic feature of angiosperms.

Xylem fibres: Xylem fibres have highly thickened walls and obliterated central lumens. They may either be septate or aseptate.

Xylem parenchyma: Xylem parenchyma cells are living and thin-walled, and their cell walls are made up of cellulose. They store food materials in the form of starch or fat, and other substances like tannins. The radial conduction of water takes place by the ray parenchymatous cells.

Primary xylem is of two types –

· Protoxylem

· Metaxylem

• The first formed primary xylem elements are protoxylem and the later formed primary xylem is metaxylem.

• In stems, the protoxylem lies towards the centre (pith) and the metaxylem lies towards the periphery of the organ. This type of primary xylem is called endarch.

• In roots, the protoxylem lies towards periphery and metaxylem lies towards the centre. Such arrangement of primary xylem is called exarch.

Phloem:

• Phloem transports food materials, usually from leaves to other parts of the plant.

Phloem tissues

Phloem in angiosperms is composed of -

· Sieve tube elements

· Companion cells

· Phloem parenchyma

· Phloem fibres

• Gymnosperms have albuminous cells and sieve cells and lacks sieve tubes and companion cells.

Sieve tube elements: Sieve tube elements are also long, tube-like structures, arranged longitudinally, associated with the companion cells with their end walls are perforated in a sieve-like manner to form the sieve plates. A mature sieve element possesses a peripheral cytoplasm and a large vacuole but lacks a nucleus. The functions of sieve tubes are controlled by the nucleus of companion cells.

Companion cells: The companion cells are specific parenchymatous cells, in close association with sieve tube elements. The sieve tube elements and companion cells are connected by pit fields present between their common longitudinal walls and these companion cells help in maintaining the pressure gradient in the sieve tubes.

Phloem parenchyma: Phloem parenchyma is made up of elongated, tapering cylindrical cells which have dense cytoplasm and nucleus. The cell wall is composed of cellulose and has pits through which plasmodesmatal connections exist between the cells. The phloem parenchyma stores food material and other substances like resins, latex and mucilage and is absent in most of the monocotyledons.

Phloem fibres: Phloem fibres (bast fibres) are made up of sclerenchymatous cells, generally absent in the primary phloem but are found in the secondary phloem. These are much elongated, unbranched, have pointed, needle like apices with thick cell wall of phloem fibres. At maturity, these fibres lose their protoplasm and become dead. Phloem fibres of jute, flax and hemp are used commercially. The first formed primary phloem consists of narrow sieve tubes and is referred to as protophloem and the later formed phloem has bigger sieve tubes and is referred to as metaphloem.