PAB 307: PLANT ANATOMY

CLASSIFICATION, DESCRIPTION AND FUNCTION OF PLANT TISSUES

General Introduction

Cells, the fundamental units of life, are associated in various ways with each other, forming coherent masses, or tissues. Thus, a tissue may be defined as a group of cells of the same or mixed type, having a common origin and performing an identical function (Dutta, 1964). Tissues composed of only one type of cell are called simple tissues, while those composed of two or more types of cells are called complex tissues. In addition, the principal tissues of vascular plants are grouped into larger units on the basis of their continuity throughout the plant body. These larger units, known as tissue systems, are readily recognized, often with unaided eye.

Classification of Tissues

Tissues may be broadly classified into two groups: meristematic and permanent.

A) Meristematic Tissues

The meristematic tissues are composed of cells in a state of division and capable of dividing. They may be classified according to their position in the plant body as apical, intercalary and lateral. According to their functions, they may be classified as protoderm, procambium and ground or fundamental meristem. The cells in these tissues are essentially alike and isodiametric. The protoplasm in them is abundant and active with large nuclei, and the vacuoles are small or absent.

B) Permanent Tissues

These are composed of cells that have lost the power of dividing, having attained their definite form and size. They may be living or dead, thin-walled or thick walled. They are formed by differentiation of cells of the meristem, and may be primary or secondary. The primary permanent tissues are derived from the apical meristems of the stem and root, and secondary permanent tissues from the lateral meristems. Permanent tissues may also be classified into simple and complex tissues, based on cell type composition. Examples of simple permanent tissues include parenchyma, collenchyma and sclerenchyma; while complex permanent tissues include xylem and phloem.

The Tissue System

On the principle of division of labour, plant tissues are arranged in three systems, each playing a definite role in the life of a plant. Each system may consist of only one tissue or a combination of tissues which may be structurally similar or of different nature, but performs a common function and have the same origin. The three systems are:

I. Epidermal tissue system II. Ground or fundamental tissue system III. Vascular tissue system

As earlier mentioned, the precursors of these tissue systems are meristems which may be outlined as follows:

Protoderm Epidermis Epidermal tissue system

Ground meristem Cortex, pericycle, medullary rays, pith Ground tissue

Procambium Vascular bundles Vascular tissue system

I. Epidermal Tissue System

The epidermal tissue system is derived from the dermatogen of the apical meristem and forms the epidermis, or outermost skin layer which extends over the entire surface of the plant body, and basically functions as a protective layer. It is continuous, except for certain openings (stomata and lenticels). The epidermis is mostly single layered, but sometimes may form multiseriate layers, and called multiple epidermis (as found in leaves of Ficus, Indian rubber plant, Piperomia, etc.). When viewed from the surface, they appear somewhat irregular in outline, varying in shape and size, but mostly appear in rectangular form in cross-section. Epidermal cells are mostly composed of parenchyma cells (parenchymatous), compactly arranged without intercellular spaces. They possess relatively small amounts of cytoplasm lining the cell wall and large vacuoles filled with colourless cell sap. The outer cell walls are unevenly thick and impregnated with cutin or suberin, while inner and radial cell walls of the epidermis are mostly thin. The cutinized outer layer (called cuticle) acts as a protective barrier, shielding inner cells against water loss, mechanical injury and potential parasites. Additional waxy substances in form of rods, scales and grains which are deposited on the cuticle help to prevent further loss of water. In many plants, the epidermal cells often bear outgrowths, known as hairs or trichomes. These may be unicellular or multicellular, simple or branched, soft or sharp and stiff. The epidermis may also bear stinging hairs, glandular hairs or dense coating of hair. The epidermis of root without cuticle and not provided with stomata is called epiblema or piliferous layer. The outer walls of most of its cells extends outwards to form tubular, unicellular prolongations called root hairs, which serve to increase absorbing surface of the root organ in the soil.

Functions of epidermis

 Protection of internal tissues against mechanical injury, injurious thermal changes, high wind velocities, attack of microbial parasites, acid rain effects, etc.  Reduces loss of water due to excessive transpiration  Presence of sharp and stiff hairs on epidermal surface of some plants protects them from grazing  Epidermis of some xerophytic plants store food and water materials in the form of mucilage  In lower plants, epidermal cells contain chloroplasts and thus manufacture food material

Special epidermal structures

1. Lithocytes: Epidermal cells of plants belonging to families like Curcubitaceae and Urticaceae contain deposits of calcium carbonate crystals, arranged in the form of bunches, like those of grapes. Such deposits are called cystoliths and the container cells are called lithocytes. Lithocytes are usually larger than other epidermal cells. 2. Stomata: These are very minute openings formed in the epidermal layer in the aerial parts of plants, mostly the leaves. A pore, the stoma, is surrounded by two specialized, chloroplast containing epidermal cells, called guard cells. The guard cells are kidney shaped in dicotyledonous plants, but dumble-shaped in monocotyledons. They function to guard the passage (stoma) by regulating the opening and closing of the stoma-like lips. Sometimes, the guard cells are surrounded by two or more cells distinct from other epidermal cells, called accessory or subsidiary cells. Stomata (pl. of stoma) occur scattered in dicot leaves, while in monocot leaves, occur in parallel rows. Under normal conditions, they remain closed at night in the absence of light and stay open during the daytime, in the presence of light. They mainly function in the interchange of gases between the plant and the atmosphere – oxygen for respiration and carbon dioxide for manufacture of carbohydrates. 3. Trichomes: These refer to different types of unicellular and multicellular extensive appendages of the epidermis which have a variety of functions. They are important for regulating moisture exchange with the atmosphere, secrete chemicals to provide defense against insect attack and also to repel animals from grazing on the plants.

II. Ground or Fundamental Tissue System This system forms the main bulk of the plant’s body and extends from below the epidermis to the centre, excluding the vascular bundles. It is heterogenous in nature, composed of different types of tissues concerned with different functions, of which parenchyma is the most abundant. The other tissues are sclerenchyma and collenchyma, and sometimes lactiferous or glandular tissue. In a stem, it basically includes cortex, pericycle, pith and medullary rays and in a leaf, mesophyll tissues are included in the system. The various sub zones of the system are outlined as follows: i. Cortex: This is the zone that lies between the epidermis and the pericycle, varying in thickness from a few to many layers. Its chief function is to act as a protective tissue in plant stems, while also performing functions of food storage and participation in metabolic activities (photosynthesis). In a typical dicot stem, it is generally differentiated into hypodermis, consisting of a few layers of collenchyma, or sometimes sclerenchyma, and situated just beneath the epidermis. The next layer is the general cortex, made up of few layers of loosely arranged, cortical parenchyma with intercellular spaces. Starch and different types of grain are usually found in these cells, while other cells such as chloroplasts, sclereids, tannin cells, resin ducts, and oil cavities may or may not be present. The innermost layer of the cortex is the endodermis, which is also known as starch sheath, as it often has numerous starch grains. It comprises a single layer of vertically elongated, modified parenchyma cells, which appear in cross-section, as barrel shaped cells without intercellular spaces. The layer is wavy in stems and often not distinguishable, while it is circular and well defined in roots. The cells are living and contain abundant protoplasm, large nuclei, and often starch grains. Some cells of the endodermis may also contain gums, tannins, etc. The endodermis is distinctly present in stems of majority of herbaceous angiosperms, aquatic plants, creepers and rhizomes. It remains absent in woody stems of dicotyledons and gymnosperms, and leaves of angiosperms. ii. Pericycle: This forms one or several cell layers found between the endodermis and vascular bundles. This region surrounds the vascular tissue and is regarded as the limiting region of the stele. It may be homogenous, consisting only of parenchymatous cells, as found in all roots and the pteridophytes. It may also be heteeogenous in nature, consisting of parenchyma and sclerenchyma, the latter forming isolated strands or patches in it. The pericycle occurs uniformly in the roots of higher plants, and is absent in roots and stems of some aquatic plants. It is single layered in the pteridophytes, but multi-layered in gymonosperms. In monocots (where secondary growth is absent), the pericycle becomes sclerenchymatous and provides mechanical strength to the stem. Other functions include being the seat of origin of lateral roots, giving rise to secondary meristems, storage of food materials, etc. iii. Pith: Also called medulla, it forms the central core of the stem and the root and is usually made of large celled parenchyma with abundant intercellular substances. In dicotyledonous stems, the pith is often large and well developed, while in the monocotyledonous stem, it is not distinguishable, owing to the scattered distribution of vascular bundles. Pith is absent in many dicotyledonous roots, whereas it is found well developed, often sclerenchymatous, in monocot roots. In the dicot stem, the pith extends outwards to the pericycle between the vascular bundles. Each extension which is a strip of parenchyma is called the pith ray or medullary ray. The cells of the pith and the pith ray are usually larger than those of the cortex and enclose numerous intercellular spaces. They basically serve to store materials such as starch, fatty substances, mucilage, etc. iv. Medullary rays: These are long strips of parenchymatous tissue existing between vascular bundles, formed from extension of pith cells occupying interfasiscular regions. Their main function is to help with conduction of food materials and water from the cortical region to pith or central portion.

Ground Tissue of leaves The ground tissue system of leaves is commonly called mesophyll tissues, which is remarkably different from that of stems and roots. In isobilateral leaves (monocot leaves with parallel venation), mesophyll tissues consist of more or less isodiametric, parenchymatous cells with intercellular spaces. In dorsi-ventral leaves (dicot leaves and concentric leaves having reticulate venation), mesophyll tissues are found differentiated into (a) Palisade cells, usually found beneath the upper epidermis (adaxial surface) and are elongated or columnar shaped, arranged at right angles with the epidermal cell layer, and (b) Spongy cells, which occur towards the lower epidermis and are characterized by isodiametric or irregular shaped cells with a large number of intercellular spaces.

The main function of the mesophyll tissue is to perform photosynthesis, and in some other plants, to store water and food materials in soluble forms. Sclereids and secretory cells may also be found in this region. They also serve to confer buoyancy to leaves of aquatic plants and some marshy plants.

III. Vascular Tissue System

This system is made of complex tissues, the xylem and the phloem, which together makes a unit strand called the vascular bundle. Vascular bundles remain distributed in stellar region. A stele is considered as a central cylinder of root, stem or leaf which consists of vascular bundles, pith and pith rays enclosed in a pericycle. The pericycle remains surrounded by endodermis. Each bundle is made up of xylem and phloem, with a cambium in dicot stems, or without a cambium in monocot stems, or of only one kind of tissue – xylem or phloem, as in roots. The function of this system is to conduct water and raw food materials from the root to the leaves, and prepared food material from leaves to the storage organs and growing regions. The vascular bundles may be regularly arranged in a ring, as in the stems of dicots, gymnosperms, and in all roots, or they may be scattered in the ground tissue, as in the stems of monocotyledons. Elements of vascular bundles

A typical primary vascular bundle of the dicot stem, when fully formed, consists of three well defined tissues (i) xylem (or wood), (ii) phloem (or bast) and, (iii) cambium.

Xylem or wood lies towards the centre of the bundle and is composed of tracheae or vessels, some tracheids, a number of wood fibres and a small patch of wood parenchyma. Vessels are of different kinds such as spiral, annular, scalariform, pitted and reticulate (with simple or bordered pits). Some tracheids also lie associated with the vessels. Wood fibres and wood parenchyma are ordinary sclerenchymatous and parenchymatous cells lying associated with the wood or xylem. Their walls are provided with simple pits. In the secondary xylem, the wood parenchyma sometimes become thick walled and lignified. Xylem vessels and tracheids are used for conduction of water and mineral salts from the roots to the leaves and other parts of the plant. The xylem parenchyma assists them in their task and also serves as food storage, and wood fibres give proper rigidity to the xylem. Except for wood parenchyma, all the other elements of the xylem are dead and lignified, hence their secondary function is to give mechanical strength to the plant.

The first formed xylem (called protoxylem) consists of annular, spiral and scalariform vessels; it lies towards the centre of the stem and its vessels have smaller cavities. The later-formed xylem (metaxylem) consists of reticulate and pitted vessels and some tracheids; it lies away from the centre and its vessels have much bigger cavities. The development of the xylem is said to be centrifugal or endarch, meaning it originates from within the stem.

Phloem or bast lies towards the circumference and consists of sieve tubes, companion cells and phloem parenchyma. The companion cells and phloem parenchyma are provided with simple pits, particularly in the walls lying against the sieve-tubes. The phloem tissue, as a whole, is used to facilitate downward movement (translocation) of prepared food materials (soluble proteins, soluble carbohydrates, and amines and amino acids) from the leaves to the storage organs, and later from there to the different growing regions in the upward direction. Sieve tubes, present in all higher plants, are the main channels through which this translocation takes place. Companion cells and phloem parenchyma, when present, assist the sieve tubes in the task. They also transmit many of the soluble food materials sideways to the surrounding tissues. All the elements of the phloem are made of cellulose and are living. Primary phloem hardly ever contains bast fibres, but it may be capped by a patch of sclerenchyma, called the hard bast, as seen in the sunflower stem. The outer portion of the phloem, consisting of narrow sieve tubes, constitutes the protophloem. The inner portion, consisting of bigger sieve-tubes, constitutes the metaphloem.

Cambium occurs as a thin strip of primary meristem cells, lying between the xylem and phloem. It consists of one or a few layers of thin-walled and roughly rectangular cells. Although cambial cells look rectangular in transverse section, they are very elongated, often with oblique ends. They become flattened tangentially, that is, at right angles to the radius of the stem.

Types of vascular bundles

Vascular bundles may be classified according to the arrangement of xylem and phloem, in the following:

1. Radial: In this type, the xylem and phloem are found as separate patches on alternate radii. These are the most primitive in nature and are characteristic of roots 2. Conjoint: The xylem and phloem, in this type, occur side by side on the same radius. According to the position of the phloem, these are recognized into two types: (a) Collateral: The collateral vascular bundles are the most common type found in stems and leaves of flowering plants. In this case, xylem and phloem lie together on the same radius, the phloem being external and the xylem being internal (towards pith) in position. The collateral vascular bundle is referred to as open type, if a strip of cambium occurs between the xylem and phloem, as is common in dicot stems, and closed type, if cambium layer is absent, as in monocot stems. (b) Bi-collateral: Two phloem patches, one on the outer side (external) and the other on the inner side (internal) of xylem, are found, as in stems of members of Cucurbitaceae. In these vascular bundles, the sequence would be outer phloem, outer cambium, xylem, inner cambium and inner phloem. These are always of the open type. 3. Concentric: In this type, one kind of vascular tissue (xylem or phloem) is completely surrounded by the other. There are two types: (i) amphivasal or leptocentric bundles, where phloem is surrounded by xylem, such as secondary vascular bundles of Dracaena and medullary bundles in dicots, and (ii) amphicribal or hadrocentric bundles, where xylem is surrounded by phloem, as in rhizomes of ferns, stems of Lycopodium and Selaginella and small bundles of flowers, fruits and some leaves of dicots. The concentric bundles are always closed. REFERENCES

1. Dutta, A.C. (1964) Botany for Degree Students. 708 pp. Sixth Edition revised in 2009 by T.C. Dutta. Oxford University Press

2. Pandey, S.N. and Chadha, A. (1996). Plant Anatomy and Embryology. 468pp. Vikas Publishing House PVT Ltd

3. Raven, P. H., Evert, R.F., and Eichhorn, S.E. (1999). Biology of Plants, Sixth Edition. W.H. Freeman and Company, Worth Publishers. New York, USA. 944pp