Tissue System of plant
About Tissue System
All the plant tissues which perform same common function, irrespective of their position in the body, constitute tissue system.On the basis of topography, the following three types of tissue systems have been recognized within the plant body by Sachs (1875).
Epidermal or dermal tissue system
Fundamental or ground tissue system
Vascular tissue system
Epidermal tissue system
The epidermal tissue system forms the outer protective covering of the plant. It is in direct contact with the external environment. In the primary plant body it is represented by epidermis and associated structures, such as trichomes, stomata, etc.
The epidermis is usually uniseriate, i.e., it is composed of a single layer of cells. But, in some plants like Ficus, Nerium, Peperomia, etc., it is multiseriate, i.e., made up of two or more layers of cells.
Structure: The epidermal cells are usually compactly arranged and do not have any intercellular spaces. They form a continuous layer interrupted by stomata. They are living cells with relatively thin walls, a large central vacuole and thin peripheral cytoplasm. The shape and size of epidermal cells vary considerably. The outer wall is usually the thickest and differs from other walls in physical and chemical characteristics due to the presence of cuticle, wax, resin, volatile gums, etc. Crystals of calcium carbonate, called cystoliths, accumulate in some specialized epidermal cells lithocytes. Lithocytes are generally larger than the adjacent epidermal cells and they usually occur in multiseriate epidermis.
In the leaves of xeric grasses, group of large epidermal cells, called bulliform cells or motor cells, are present. During dry season, loss in turgidity of these cells brings about rolling of the leaf upward and inward. This helps in reducing transpiration.
The root epidermis is commonly known as epiblema or piliferous layer or rhizodermis. Some cells (trichoblasts) of this layer give rise to unicellular tubular extensions known as root hairs. They help in the absorption of water and mineral nutrients from the soil.
In the aerial roots of some orchids and other epiphytic plants, a specialized water absorbing tissue is present which is called as velamen. The cells of velamen are compactly arranged and are dead. Some of these cells have secondary wall thickening. The main function of velamen is to absorb moisture from the atmosphere. But it is also considered as a protective layer which checks evaporation of water from cortical cells.
Structure: Stomata are minute apertures in the epidermis of leaves and other aerial parts of the plant. Each aperture is bound by two specialized kidney shaped cells, called guard cells. The inner wall of guard cells, which faces the aperture, is highly thickened, while the wall away from the aperture is thin and extensible. Mitochondria, dictyosomes, ribosomes and abundant endoplasmic reticulum are present in guard cells. They also contain chloroplasts.Guard cells are surrounded by a variable number of epidermal cells which are called subsidiary or accessory cells. These cells may be morphologically similar to the outer epidermal cells or differ from them. The two guard cells, its aperture and the adjoining subsidiary cells together constitute a stoma or stomatal apparatus.In some monocots like Cynodon dactylon, Zea mays, etc., guard cells are dumb-bell shaped. The guard cells are living and contain chloroplasts.
Functions: The major roles of stomata are as follows :
They are avenues for gaseous exchange and hence they play important role in respiration and photosynthesis.
Most of the water absorbed by the plant is transpired through stomata.
Trichomes (Epidermal Hairs)
Trichomes are unicellular or multicellular appendages of the epidermis and these are found in all plant parts. They are highly variable in structure, form and function.
They may be stellate or glandular or floccose type, etc.
Thrichomes help in checking of excess loss of water, and provide protection.
Fundamental or ground tissue system
The fundamental or ground tissue system forms the main bulk of the plant body and it extends from below the epidermis to the centre, excluding vascular bundles.In roots and dicotyledonous stems, the ground tissue present outside the vascular cylinder is known as extra stelar ground tissue and that present inner to the vascular cylinder is said to be intrastelar ground tissue.The intrastelar ground tissue includes pericycle, medullary rays and pith. However, in monocotyledonous stems, where vascular bundles are scattered, there is no such distinction in the ground tissue system. The fundamental (ground) tissue system can be distinguished into cortex, pericycle, pith or medulla and medullary rays.
The ground tissue in the extrastelar region is known as cortex. In dicotyledonous stems, cortex is usually differentiated into three regions: hypodermis, general cortex and endodermis:
The outermost region of the cortex which lies just below the epidermis is called hypodermis. It consists of 3-5 or more layers of collenchymatous (e.g., dicot stems) or sclerenchymatous cells (e.g., monocot stems). Hypodermis provides mechanical strength to the plant.
The zone of tissue between the hypodermis and endodermis constitutes general cortex. It is multilayered zone composed of parenchymatous cells with prominent intercellular spaces.
This region provides mechanical support and also stores food material.
ENDODERMIS (STARCH SHEATH)
Endodermis is the inner limiting layer of the cortex. It is a single layer of compactly arranged, vertically elongated cells which appear barrel shaped in cross section.In roots, endodermis is a well defined layer with characteristic wall thickenings in the form of Casparian bands (named after R. Caspary who discovered them in 1865), but in stems, it is ill defined or altogether absent. As endodermal cells often contain starch grains, this layer is also called starch sheath.In roots, some cells of the endodermis, usually the ones opposite to protoxylem remain thin walled. These cells are called passage cells, and they allow radial diffusion of water.Endodermis has been considered as mechanical protective layer, a layer connected with the maintenance of root pressure, and an air dam preventing the water conducting cells from becoming clogged with air, Recent studies, however, suggest that endodermis is regulatory in functions. It is a check post that prevents leakage of nutrients from the vascular tissue and regulates inflow of water with mineral salts absorbed by root through cortex directly into xylem elements.
It follows the endodermis and constitutes the outer boundary of the primary vascular cylinder. In roots, it is usually a complete ring of cells. It is absent in roots of parasitic plants and hydrophytes. In Smilax root, pericycle is multilayered and sclerenchymatous.In dicot stems, it is usually multilayered and may be parenchymatous (most of dicots), sclerenchymatous (cucurbits) or both (Asteraceae).
vascular tissue system
Central column of axis (root and stem) is called stele, which is made of number of vascular bundles, which constitute vascular tissue system.The vascular bundles are having xylem, phloem and cambium (if present).It consists of a variable number of vascular bundles which are arranged in a ring in roots and dicotyledonous stems, and are scattered in general ground tissue in monocotyledonous stem.The vascular bundles conduct water and raw food materials from roots to leaves and prepared food materials from leaves to storage organs and growing regions.
The first formed xylem elements are known as protoxylem and the later formed metaxylem. On the basis of the position of protoxylem in the vascular bundle, the primary xylem may be:Exarch: In exarch condition the protoxylem is towards the periphery and the metaxylem toward the centre of the axis. This condition of xylem is found in roots. The mode of development of xylem is centripetal.
Endarch: In endarch condition the protoxylem is towards the centre of the axis and the metaxylem towards the periphery. Such a condition of xylem occurs in stems. The mode of development of xylem is centrifugal.
Mesarch: In mesarch condition the protoxylem develops at the middle of pericycle and pith. Now it grows on both sides to form metaxylem. Thus protoxylem becomes median to metaxylem. This condition occurs in rachis of ferns and rachis and leaflets of Cycas. The mode of development of xylem is diploxylic.
The first formed sieve elements are known as protophloem and the later formed metaphloem. The sieve elements of protophloem are narrow and less distinct, whereas those of metaphloem are broad and distinct.
The cells of procambium which lie in between xylem and phloem and remain meristematic constitute the vascular cambium. Thus, cambium is a residual meristem of procambium. The cells of vascular cambium are rectangular, thin walled and highly vacuolated unlike the other meristematic cells.The cambial cells mostly divide periclinally; after each division one of the daughter cells forms secondary xylem or phloem element, while the other retains its meristematic, properties.Vascular cambium is usually composed of two basic types of initials:
Fusiform initials: They are considerably long cells with tapering ends and owe their name to the spindle shape they show in tangential section. Secondary xylem and secondary phloem are differentiated from these initials.
Ray initials: They are essentially isodiametric cells and give rise to parenchymatous secondary medullary rays.
Types of Vascular Bundles
On the basis of the presence or absence of cambium, following two types of vascular bundles have been recognized:
Open bundle: In these vascular bundles cambium is present in between xylem and phloem. They are characteristic of dicotyledonous stem and are capable of undergoing secondary growth.,
Closed bundle: These vascular bundles are without cambium. These are the characteristic feature of monocotyledonous stems and are incapable of undergoing secondary growth.
On the basis of relative position of xylem and phloem, the following four types of vascular bundles have been recognized.
Radial vascular bundles : When xylem and phloem groups are arranged alternately on different radii, such type of vascular bundles are known as radial. The radial bundles are characteristic features of roots.
Collateral: In these bundles, xylem and phloem are present on the same radius and the phloem is located on the outer side of the xylem. In dicotyledonous stems, collateral bundles are of open type (intrafascicular cambium present), whereas in monocotyledonous stems they are of closed type (cambium absent).
Bicollateral: In the stems of Solanaceae, Cucurbitaceae, Apocynaceae, etc. phloem also differentiates on the inner side of the xylem and this phloem is known as internal phloem. A vascular bundle with both external and internal phloems is said to be bicollateral vascular bundle. Such vascular bundle is always open.
Concentric: In concentric bundles one type of vascular tissue (xylem or phloem) is surrounded by the other. A concentric bundle is always closed. Concentric vascular bundles are of two types:
Amphivasal (leptocentric): In these bundles phloem is surrounded all around by xylem. Amphivasal bundles occur in some monocot stems such as Dracaena and Yucca.
Amphicribal (hadrocentric): In these bundles xylem is surrounded all around by phloem. Amphicribal bundles occur in ferns.