Different types of Plant tissue
Tissue is a group of similar or dissimilar cells having common origin and responsible for performing similar function. The plant tissues can be categorised into two groups, i.e., meristematic tissue and permanent tissue. These are as follows
Meristematic tissue is composed of immature young cells capable of repeated divisions. The meristematic cells are thin walled, spherical, oval, or polygonal in shape, densely filled with protoplasm, have large nuclei and are actively metabolising. The cells have the capacity of division. They grow and form the plant body
other than embryonic cells.
Meristematic tissues are of different types based on different sources
On the Basis of Origin
Plant tissue is of two types
(i) Primary Present from beginning and persist throughout life, responsible for primary growth, lie mainly in the apices of stem and roots hence, add to length of roots and shoots.
(ii) Secondary Arise amongest and from the non-meristematic tissue by dedifferentiation of permanent cells at later stages in life, e.g., cork
cambium, and inter-fascicular cambium.
On the Basis of Location
Plant tissue is of three types
(i) Apical Primary meristem located in growing points (root and shoot apex) responsible for primary growth, results in increase in length.
(ii) Lateral Situated in older parts, parallel to longof organ. These divide periclinally and responsiblefor increase in girth (secondary growth), e.g, cork
cambium, and vascular cambium
(iii) Intercalary Located between regions of permanent tissues (e.g., at nodes), responsible for growth in length in region other than tip.
On the Basis of Plane of Division
Plant tissue is of three types
(i) Mass meristem Cell division occurs in all planes
leading to increase in volume, e.g, development of endosperm, and embryo.
(ii)Plate meristem Cell division occurs only on two planes showing increase in area, e.g., epidermis.
(iii)Rib or file meristem Cells divide only on one plane increasing the length, e.g., development of cortex and pith.
On the Basis of Function
Plant tissue is of three types
(i) Protoderm Outermost meristematic layer of apical meristem which develops into epidermis lateron.
(ii) Procambium Located inside protoderm and gives rise to primary vascular tissue.
(iii)Ground meristem Major part of apical meristem which is precursor of ground tissue such as pith,cortex,hypodermis and pericycle.
Root Apex Organisation
(i) Root apex consists of mass of meristematic cells. Due to presence of root cap the root meristem becomes subterminal in position.
(ii) The theories given for root apex organisation are
(a) Histogen theory According to this theory given by Hanstein (1870), the root apices consist of central core called plerome, surrounded by several layers of periblem, outermost single layer of dermatogen and calyptrogen. The calyptrogen gives rise to root cap, dermatogen to epidermal tissue system, periblem to ground tissue system and the plerome to vascular tissue system.
(b) Korper-Kappe theory It was propounded by Schuepp (1917). According to this theory, the cells in root apex divide in two planes called T division.
(c) Quiescent centre theory This theory was proposed by Claws (1956-58) in maize. According to this theory, root apex consists of an inverted cup-like region the quiescent centre. The cells of this region have low amount of RNA, DNA and protein and have low dividing activity. These are reserve meristem.
Shoot Apex Organisation
Shoot apex is present immediately above the youngest leaf primordia. It consists of meristematic cells.
The theories put forward to explain shoot apex organisation are
★ Apical cell theory This theory was proposed by Nagelli (1858). According to this theory, a single apical cell leads to development of entire plant body.
★Histogen theory This theory was proposed by Hanstein (1870). According to this theory, shoot apex consists of meristematic zones the histogens, which are dermatogen [forms epidermal tissue system), periblem (forms ground tissue system) and plerome [forms vascular tissue system).
★Tunica corpus theory This was proposed by Schmidt (1924). According to this theory, shoot apex consists of tunica (which forms epidermis) and corpus (which forms central mass).
These tissues are made up of cells that have lost the power of division and are differentiated to carry out various functions. The cells are living or dead.
Permanent tissues are of three types, i.e., simple, complex and special (secretory).
It is homogenous in nature and composed of similar cells. It is of three types
i) Parenchyma Composed of living, isodiametric, spherical, oval or polygonal cells with intercellular spaces and thin cellulosic cell wall. The main function is storage of food.
The parenchyma storing ergastic substances like tannins, resins and gum are called idioblasts. The parenchyma having large intercellular spaces are called aerenchyma. The parenchyma having chloroplast are called chlorenchyma which helps in photosynthesis.
(ii) Collenchyma Consists of living cells but cellulose and pectic substances are deposited at the corners of cells, present in outer region of cortex of stem, leaves, petiole of dicot plants and absent from roots and monocot plants. It is mechanical tissue which provides, support, flexibility and elasticity to the organ.
(iii) Sclerenchyma Consists of dead cells with thick lignified cell walls, provide support and mechanical strength. It consists of the following two parts
(a) Sclereids (stone cells) Lignified extremely thick walled, so that the lumen of the cells is almost oblitrated and may be spherical, oval, cylindrical, T-shaped or even stellate. Found in hard parts of plant, e.g., endocarp of walnut and coconut.
(b) Sclerenchymatous fibres They are long and tapering at ends. The cells are lignified, and dead commonly found in pericycle of stem forming a solid tissue protecting the vascular bundles of dicots.
It is composed of more than one type of cells.
It is of two types.
(i) Xylem Forms the bulk of roots and stems, conducting water and minerals and provide support it is composed of
(a) Tracheids Elongated, tube-like, dead, single cells with hard, thick and lignified walls. End walls are tapered and overlap with adjacent tracheids. Water passes tracheid to tracheid through the pits in their walls.
(b) Vessels Conducting unit of xylem, long, tubular structure formed by end to end fusion of several cells. The first vessels form protoxylem. Lignin is deposited in the cellulose wall of these vessels so as to form annular or spiral thickenings and forming metaxylem. The long vessels transport large amount of water and minerals over long distances.
(c) Xylem parenchyma Living, thin-walled abundant parenchymatous cells, involved in storage of starch, sugar and lipids.
(d) Xylem fibres Sclerenchymatous, occur abundantly in woody dicots. Have thicker wall and narrower lumens. Stronger and provide mechanical support.
(ii) Phloem Also called bast, main function is transport of food. It is composed of Sieve tubes Long, slender, tube-like structures involved in transport of organic solutes.
Formed by end to and fusion of sieve tube-elements. The wall of sieve tube elements are made up of cellulose and pectic substances but the nuclei degenerate at maturity.
Two adjoining end walls of neighbouring sieve elements form sieve plate. Remain alive by depending upon companion cell.
Companion cells A thin-walled elongated cell is associated with each sieve tube and are connected by simple pits. Companion cell is living, contains dense protoplasm and large elongated nucleus.
Phloem parenchyma Composed of living and cylindrical cells. These are absent in most of the monocots and mainly store food material.
Phloem flbres Sclerenchymatous and provide mechanical support. Phloem fibres of jute, flax and hemp are retted in water and used for making ropes and coarse textiles.
Special or Secretory Tissue
Plant tissue is of two types
(i) Laticiferous tissue Made up of thin walled, elongated, branched and multinucleate structures that contain colourless, milky or yellow coloured juice called latex.
(a) Latex cells These do not fuse hence, not form network. Plants having such tissues are called simple or non-articulated laticifers.
e.g., Calotropis, Nerium, Vinca, Euphorbia, Ficus,
(b) Latex vessels Formed due to fusion of cells and form network-like structure.
The plants are called compound or articulated laticifers.
e.g., Argemone, Papaver, Sonchus.
(ii) Glandular tissue Include different type of glands which secrete oils, gums, mucilage, tannins and resins.
It is of two types
(i) External glands Generally occur on the epidermis of stem and leaves, as glandular hair in Plumbago and Boerhauvia, stinging hair secrete poisonous substance in Urtica diosica, nectar secreting glands in flowers, digestive enzyme secreting glands in insectivorous plants.
(ii) Internal glands Present internally, e.g., oil glands in Citrus and Eucalyptus, resinous ducts in Pinus and mucilage canals in Cycas. The glands secreting essential oil are called osmophores.
Types of Vascular Bundles
The vascular bundles found in stelar part constitute vascular tissue system. Xylem, phloem and cambium are main parts of vascular bundle.
The vascular bundle may be of foIIowing types
(i) Radial The xylem and phloem are arranged on different radii alternating with each other, e.g., roots.
(ii) Conjoint Xylem and phloem present in the same vascular bundle and on the same radius, e.g., stem. They may be
(a) Collateral Xylem is towards innerside and phloem towards outside.
(b) Bicollateral Phloem present on both sides of xylem, e.g., Cucurbita.
(iii) Concentric One vascular tissue surrounds the other. They may be
(a) Anlphicribal or Hadrocentric Xylem is surrounded on all sides by phloem, e.g., ferns.
(b) Amphivasal or Leptocentric Phloem is surrounded on all sides by xylem, e.g., Yucca, Dmaaena.
When cambium is present between xylem and phloem the vascular bundle is called open (e.g., dicot stem) and when cambium is absent, the vascular bundle is called closed (e.g., monocot stem)