All the tissues of a plant which perform the same usual function regardless of position or continuity in the body may be considered to produce, together a tissue system.
There are three major tissue systems:
1. The dermal or epidermal tissue system,
2. The vascular tissue system and
3. The fundamental or ground tissue systems.
1. The Dermal or Epidermal Tissue System:
The dermal system produces the outer protective covering of the plant and is represented, in the primary plant body, by epidermis. During secondary growth the epidermis can be replaced by another dermal system, the periderm, with the cork cells forming the new protective tissues.
i. The Epidermis:
The epidermis usually consists of a single layer of cells which cover the whole outer surface of the plant body. The word is derived from two words of Greek origin, epi, upon and derma, skin. It is a continuous layer except for some small pores, called stomata and lenticels.
According to the histogen theory, it is derived from dermatogen of apical meristem. Mostly the epidermis is single layered, but in several plants it has been described as bi-or multiseriate. In the leaves of India rubber plants (Ficus elastica), banyan tree (Ficus bengalensis), oleander (Nerium spp.) etc., it becomes two to multilayered. The epidermal cells can be somewhat irregular in outline usually varying in shape and size and arranged very close to each other having no intercellular spaces among them.
In the case of roots the outermost layer is called the epiblema, piliferous layer or rhizodermis. Usually its cells extend outwards in the produce tubular unicellular root hairs, which help in the absorption of water and mineral nutrients from the soil.
Functions of the Epidermis:
(a) The epidermis is primarily a covering layer which helps in protection of the internal soft tissues against mechanical injury.
(b) It prevents excessive evaporation of water from the internal tissues, for this, many adaptations like development of thick cuticle, wax, hairs etc., take place.
(c) It also serves in photosynthesis and secretion.
(d) The epidermis acts as store house of water in many xerophytic plants.
(e) Certain of the epidermal cells develop into the secretory tissues of nectaries, the stomata of leaves and stem, and the absorbing hairs of roots.
ii. The Stomata:
The stomata are minute pores which exist in the epidermis of the plants. Each stoma remains surrounded by two kidneys or bean shaped epidermal cells, the guard cells. The stomata may exist on any part of a plant except the roots. The epidermal cells bordering the guard cells are called accessory cells or subsidiary cells. Generally the term stoma is applied to the stomata opening and the guard cells.
The guard cells are living and possess chloroplasts in them. They also contain a larger proportion of protoplasm than other epidermal cells. Usually in the leaves of dicotyledons the stomata remains scattered whereas in the leaves of monocotyledons they are arranged in parallel rows.
The number of stomata can also range on the surface of single leaf from a few thousand to hundreds of thousands per square centimetre. Stomata occur on both upper and lower surfaces of leaf, but especially they are confined to the lower surface. In floating leaves stomata are confined only on the upper surface of the leaf. Under normal conditions the stomata remain closed in the lack of light or in night or remain open in the presence of light or in day time.
The gramineous stoma contains guard cells of which the middle portions are much narrower than the ends so that the cells appear in surface view like dumbbells. They are commonly observed in Gramineae and Cyperaceae of monocotyledons.
Functions of Stomata:
They are used for the exchange of gases in between the plant and atmosphere. To facilitate this function, each stoma opens in a sub-stomata chamber of respiratory cavity. Evaporation of water also takes place through stomata.
2. The Vascular Tissue System:
The vascular tissue system includes a number of vascular bundles which are found to be distributed in the stele. The stele is the central cylindrical portion of the stem and the root, generally surrounded by the endodermis, and consists of vascular bundles, pericycle, pith and medullary rays. Each vascular bundle consists of xylem and phloem tissues with or without cambium. In roots, separate xylem and phloem strands are observed.
The function of this system is to conduct water and other nutrients from roots to leaves through the xylem and transaction of prepared carbohydrates from leaves to other storage organs and growing regions of plant body through the phloem.
The vascular bundle elements are derived from the procambial strands of the primary meristem. The vascular bundles can be arranged in circular ring as in the dictotyledonous stems and the roots, on the other hand they are found to be scattered throughout the axis in the monocotyledonous stems.
Constituents of a vascular bundle:
A vascular bundle of dicotyledonous stem includes three major zones:
(a) Xylem or wood,
(b) Phloem or bast, and
(c) Cambium.
(a) Xylem or Wood:
The xylem of a vascular bundle lies towards the centre and is made- up of:
(i) Vessels or tracheae,
(ii) Tracheids,
(iii) Wood fibres, and
(vi) A patch of xylem or wood parenchyma.
The first cells of xylem to mature are collectively known as the protoxylem. The protoxylem is a complex tissue made up of trancheids, vessels and parenchyma cells. The protoxylem includes annular, spiral and scalariform vessels which may stretch in length very easily. In the stems it lies towards the centre of the axis whereas in the roots it lies towards periphery.
The vessels of protoxylem have smaller cavities. The xylem which evolves afterwards and possesses reticulate and pitted vessels and some tracheids is called metaxylem. In stems the metaxylem is always observed towards periphery whereas in the roots towards the centre. The vessels of metaxylem have bigger and wider cavities.
(b) Phloem or bast:
Usually in stems, phloem is observed away from the centre of the axis towards the periphery and includes sieve tubes or sieve cells only, or sieve tubes and companion cells only, or sieve-tubes, companion cells and phloem parenchyma. In gymnosperms it is represented by sieve cells only. In most of monocotyledons it includes sieve tubes and companion cells only, whereas in dicotyledons, sieve tubes, companion cells and phloem parenchyma are found together.
The first cells of the phloem to mature are called protophloem. The protophloem consists of narrow sieve tubes, and is found towards periphery. The inner portion of the phloem consists of bigger sieve tubes called metaphloem. The metaphloem is complex tissue and includes well developed cells of all types such as—sieve tubes, companion cells, phloem parenchyma and sometimes phloem fibres and sclereids.
(c) Cambium:
In between xylem and phloem, a thin strip of primary meristem is observed in dicotyledonous stems, termed the cambium. The cells of cambium are rectangular and thin- walled. The cambium strip can be uniseriate or multi-layered. The cambial cells are living, sufficiently elongated and possess oblique ends, but as they become flattened tangentially they look rectangular in cross section.
3. The Fundamental or Ground Tissue System:
Cortex:
The ground tissue observed beneath the epidermis which surrounds the central cylinder and is delimited from the cylinder by the endodermis is termed the cortex. Usually the cortex of stems consists of thin-walled parenchyma cells having sufficiently developed intercellular spaces among them. Generally some of the cortical cells or all of them contain chloroplasts at least in young stems.
The cortical cells also contain starch, tannins, crystals and other common secretions in them. The cortex can contain collenchyma, sclerenchyma and sclereids in addition to ordinary parenchyma. Collenchyma is arranged as a cylinder or in the form of strands near to beneath the epidermis.
In most of the dicotyledonous stems collenchyma is often found in the ridges, in the corners and in other portions to give temporary support to the plant body. Sometimes a few layers of fibres or collenchyma evolve just beneath the epidermis forming an outer protective layer called hydpodermis. Just beneath the hypodermis a few layers of parenchyma and chloroenchyma are observed.
The innermost layer of the cortex is endodermis which is single layered and sometimes known as starch seath. The cortex of roots is more homogenous than that of stems and usually includes parenchyma only.
Function of the Cortex:
In stems it acts as a protective tissue, but secondarily carbon assimilation, storage of water, storage of food and other functions are also carried on. Collenchyma of the cortical region aids in the temporary mechanical support of plant body. In roots it is a storage tissue and helps in pumping water from hairs to the xylem.
Types of Vascular Bundles:
A vascular bundle includes a strand-like portion having xylem and phloem of the primary vascular system.
According to the arrangement of xylem and phloem in the vascular bundles, they are being arranged in the following major types:
(a) Radial,
(b) Conjoint, and
(c) Concentric.
a. Radial:
Those, in which the xylem and the phloem lie radially side by side (viz., in roots of seed plants). This is most primitive type.
b. Conjoint:
Those in which the two types of tissues are separated from one another, here xylem and phloem are together form a bundle.
The two subtypes are:
(i) Collateral, and
(ii) Bicol- laterlal.
(i) Collateral:
The xylem and phloem lie together on the same radius in the position that xylem lies inwards and the phloem outwards. Here the phloem exists on one side of the xylem strand. In dicotyledonous stem, the cambium is found to be present in between xylem and phloem, such bundles are known as open (e.g., in Helianthus), and when the cambium is absent it is called closed (e.g., in monocotyledonous stems).
(ii) Bicollateral:
In such bundles the phloem is observed to be present on both sides of xylem. Simultaneously two cambium strips also occur. Various elements are arranged in the following sequence— outer phloem, outer cambium, xylem, inner cambium and inner phloem. Such bundles are commonly observed in the members of Cucurbitaceae.
Such bundles are always open.
c. Concentric:
Those, in which one type of tissue surround or ensheaths, the other, the concentric bundles can be of two subtypes, amphviasal and amphicribral. If the xylem surrounds the phloem it is known as amphivasal bundle as found in Dracaena, Yucca and other monocots and some dicots. If the phloem surrounds the xylem, it is amphicribal as found in many ferns. These bundles are always closed.
Nodal Anatomy:
Leaf Traces and Leaf Gaps:
A shoot bears nodes and internodes. At each node, portions of the vascular system are deflected into the leaf, which is attached at this node. A vascular bundle situated in the stem but directly related to a leaf, to represent the lower part of the vascular supply of such leaf, is termed the leaf trace.
The leaf trace is defined as follows-The leaf trace is vascular bundle that connects the vascular system of the leaf with that of the stem. A leaf trace is extended between the base of a leaf and the point where it is entirely merged with other parts of the vascular system in the stem. One or more leaf traces may be associated with each leaf.
In the shoot of pteropsid (seed plants and ferns) where the leaf trace diverge into a leaf, it appears as though a portion of the vascular cylinder of the stem is deflected to one side. Immediately above the diverging trace, a parenchymatous tissue is being differentiated instead of vascular tissue in the vascular region of the stem for a limited distance.
The parenchymatous regions in the vascular system of the stem, situated adaxially from the diverging leaf traces, are called leaf gaps or lacunae. Actually these gaps are not breaks in the continuity of the vascular system of the axis. Lateral connections exist between the tissues above and below the gap. In transverse sections of an axis at level of a leaf gap, the gap resembles an interfascicular area.
The gaps are quite conspicuous in the fern and angiosperms where the vascular system in the internodal parts of the stem forms a more or less continuous cylinder. In certain ferns the leaves are so crowded that the gaps formed at the successive nodes overlap one another and the vascular cylinder appears highly dissected.
No comments yet.