This chapter introduces to the internal structure ie anatomy and functional organisation of higher plants.
Plants have cells as the basic unit, cells are organised into tissues and in turn the tissues are organised into organs. Organs are organised into organ systems.
Different organs in a plant show differences in their internal structure. Within angiosperms, the monocots and dicots are also seen to be anatomically different.
Internal structures also show adaptations to diverse environments.
6.1 THE TISSUE SYSTEM
The types of tissues, their structure and functions in a plant is based on the types of cells present in them, their location in the plant body.
On the basis of their structure and location, there are three types of tissue systems:
Epidermal tissue system
Ground or fundamental tissue system
Vascular or conducting tissue system
6.1.1 Epidermal Tissue System
Epi - above, Derma - skin
It forms the outer-most covering of whole plant body and comprises of:
Epidermal cells
Stomata and
Epidermal appendages – trichomes and hairs.
Epidermal cells
These are parenchymatous (typically composed of living cells that are thin-walled, unspecialized in structure, and therefore adaptable, with differentiation, to various functions) with a small amount of cytoplasm lining the cell wall and a large vacuole.
The outside of the epidermis is often covered with a waxy thick layer called the cuticle which prevents the loss of water.
Cuticle is absent in roots.
Stomata are structures present in the epidermis of leaves.
Stomata regulate the process of transpiration (process of water passing out from the surface of a plant or leaf) and gaseous exchange.
Each stoma is composed of two bean shaped specialised cells known as guard cells which enclose stomatal pore. In grasses, the guard cells are dumb-bell shaped. These are present in the epidermis of leaves, stems and other organs that are used to control gas exchange.
The outer walls of guard cells (away from the stomatal pore) are thin and the inner walls (towards the stomatal pore) are highly thickened.
The stomatal pores are largest when water is freely available and the guard cells become turgid (swollen/ tight), and closed when water availability is critically low and the guard cells become flaccid (loose).
The guard cells possess chloroplasts and regulate the opening and closing of stomata.
Light is the main trigger for the opening or closing of stomata. When light intensity is high, potassium ions move into guard cells increasing their concentration. This causes water to move into the guard cells from more dilute areas of mesophyll tissue by osmosis. The additional water leads to the guard cells swelling unevenly because the thicker, inner walls are less flexible than the thinner, outer walls. Thus it leads to opening of stomata by turgor/ osmatic pressure.
This is when, CO2 from air enters through the stomata into the mesophyll tissues. O2 is produced as a by product of photosynthesis, exits the plant via the stomata.
When there is no light, the sugar in guard cells turns to starch. The starch is insoluble, so the guard cells have a lower concentration than the neighboring cells, and the neighboring cells osmotically remove the water from the guard cells, rendering them flaccid and the stomata close.
Also stomata plays a vital role in the Transpiration, the process of water movement through a plant and its evoporation from aerial parts, such as leaves, stems and flowers. It is a passive process that requires no energy (passive) expense by the plant. It also cools plants, changes osmatic pressure of cells, and enables mass flow of mineral nutrients. When water uptake by the roots is less than the water lost to the atmosphere by evaporation plants close small pores called stomata to decrease water loss, which slows down nutrient uptake and decreases CO2 absorption from the atmosphere limiting metabolic processes, photosynthesis, and growth.
In xerophytes the stomata open in night and remain closed during day as an adaptation to conserve water.
Sometimes, a few epidermal cells, in the vicinity of the guard cells become specialised in their shape and size and are known as subsidiary cells. The stomatal aperture, guard cells and the surrounding subsidiary cells are together called stomatal apparatus (Figure 6.1).
The cells of epidermis bear a number of hairs. The root hairs are unicellular elongations of the epidermal cells and help absorb water and minerals from the soil.
On the stem the epidermal hairs are called trichomes. The trichomes in the shoot system are usually multicellular. They may be branched or unbranched and soft or stiff. They may even be secretory. The trichomes help in preventing water loss due to transpiration.
6.1.2 The Ground Tissue System
All tissues except epidermis and vascular bundles constitute the ground tissue.
It consists of simple tissues such as parenchyma, collenchyma and sclerenchyma.
Parenchymatous cells are usually present in cortex , pericycle, pith and medullary rays, in the primary stems and roots. In leaves, the ground tissue consists of thin-walled chloroplast containing cells and is called mesophyll.
Cortex is tissue of unspecialized cells lying between the epidermis and the vascular/ conducting tissues of stems and roots. Cortical cells may contain stored carbohydrates or other substances such as resins, latex, essential oils, and tannins. In roots and in some herbaceous stems but not usually in woody stems, the innermost layer of cortical cells is differentiated into a cell layer called the endodermis.
Pericycle cells form an outer cell layer that surrounds and protects cylindrical bundles containing vascular xylem and phloem cells. These are found in the center of roots and shoots. The pericycle cells also help xylem and phloem cells in transport of nutrients and water. Pericycle cells also play important functional roles in lateral root growth and secondary growth.
Pith, or medulla, is a tissue in the stems of vascular plants. It is composed of soft, spongy parenchyma cells, which in some cases can store starch. In eudicots, pith is located in the center of the stem. In monocots, it extends also into flowering stems and roots. The pith is encircled by a ring of xylem; the xylem, in turn, is encircled by a ring of phloem.
MEDULLARY RAYS is a primary tissue in the stems of vascular plants composed of radiating bands of parenchyma cells extending between the vascular bundles of herbaceous dicotyledonous stems and connecting the pith with the cortex.
6.1.3 The Vascular Tissue System
It consists of complex tissues, the phloem and the xylem. They together constitute vascular bundles (Figure 6.2).
In the monocotyledons, the vascular bundles have no cambium present in them. Hence, since they do not form secondary tissues they are referred to as closed.
In radial type of vascular bundles, xylem and phloem are arranged in an alternate manner within the vascular bundle along the different radii such as in roots.
In conjoint type of vascular bundles, the xylem and phloem are jointly situated along the same radius of vascular bundles such as in stems and leaves. The phloem is located only on the outer side of xylem.
6.2 ANATOMY OF DICOTYLEDONOUS AND MONOCOTYLEDONOUS PLANTS
6.2.1 Dicotyledonous Root
The internal tissue organisation is as follows:
Epidermis:
It is the outermost layer.
Many of its cells protrude in the form of unicellular root hairs.
Epidermis along with root hairs is known as epiblema or rhizodermis.
Cortex:
It consists of several layers of thin-walled parenchyma cells with intercellular spaces.
Its innermost layer comprises of a single layer of barrelshaped cells without any intercellular spaces called endodermis.
The tangential as well as radial walls of the endodermal cells have a deposition of water-impermeable, waxy material suberin in the form of casparian strips.
Pericycle:
These are a few layers of thick-walled parenchyomatous cells laying inside next to endodermis.
Initiation of lateral roots and vascular cambium during the secondary growth takes place in these cells.
Pith: It is small or inconspicuous.
Conjuctive tissue: It is formed by the parenchymatous cells laying between the xylem and the phloem.
Xylem and phloem: There are usually two to four xylem and phloem patches. Later, a cambium ring develops between the xylem and phloem.
Stele: All tissues on the innerside of the endodermis such as pericycle, vascular bundles and pith constitute the stele.
6.2.2 Monocotyledonous Root
The anatomy of the monocot root is similar to the dicot root in many respects (Figure 6.3 b).
As compared to the dicot root which have fewer xylem bundles, there are usually more than six (polyarch) xylem bundles in the monocot root. Pith is large and well developed. Monocotyledonous roots do not undergo any secondary growth.
6.2.3 Dicotyledonous Stem
The transverse section of a typical young dicotyledonous stem shows that the epidermis is the outermost protective layer of the stem (Figure 6.4 a). Covered with a thin layer of cuticle, it may bear trichomes and a few stomata.
Cortex consists of three sub-zones.
Hypodermis: It is the outer most sub-zone of cortex consisting of a few layers of collenchymatous cells just below the epidermis, which provide mechanical strength to the young stem.
Parenchyma: These are the cortical layers below hypodermis consist of rounded thin walled parenchymatous cells with conspicuous intercellular spaces.
Endodermis: It is the innermost layer of the cortex. The cells of the endodermis are rich in starch grains and the layer is also referred to as the starch sheath.
Pericycle is present on the inner side of the endodermis and above the phloem in the form of semi-lunar patches of sclerenchyma.
In between the vascular bundles there are a few layers of radially placed parenchymatous cells, which constitute medullary rays.
A large number of vascular bundles are arranged in a ring ; the ‘ring’ arrangement of vascular bundles is a characteristic of dicot stem. Each vascular bundle is conjoint, open, and with endarch protoxylem (the arrangement where protoxylem (older xylem) is inside and metaxylem (newly formed) is outside) .
A large number of rounded, parenchymatous cells with large intercellular spaces which occupy the central portion of the stem constitute the pith.
6.2.4 Monocotyledonous Stem
The monocot stem has a sclerenchymatous hypodermis, a large number of scattered vascular bundles, each surrounded by a sclerenchymatous bundle sheath, and a large, conspicuous parenchymatous ground tissue (Figure 6.4 b).
Peripheral vascular bundles are generally smaller than the centrally located ones.
The phloem parenchyma is absent, and water-containing cavities are present within the vascular bundles.
6.2.5 Dorsiventral (Dicotyledonous) Leaf
The vertical section of a dorsiventral leaf through the lamina shows three main parts, namely, epidermis, mesophyll and vascular system.
The abaxial epidermis generally bears more stomata than the adaxial epidermis. The latter may even lack stomata.
The tissue between the adaxial/ upper and the abaxial/ lower epidermis is called the mesophyll.
Mesophyll, which possesses chloroplasts and carry out photosynthesis, is made up of parenchyma.
Mesophyll has two types of cells – the palisade parenchyma and the spongy parenchyma.
The adaxially placed palisade parenchyma is made up of elongated cells, which are arranged vertically and parallel to each other.
The oval or round and loosely arranged spongy parenchyma is situated below the palisade cells and extends to the lower epidermis. There are numerous large spaces and air cavities between these cells.
Vascular system includes vascular bundles, which can be seen in the veins and the midrib. The size of the vascular bundles are dependent on the size of the veins. The veins vary in thickness in the reticulate venation of the dicot leaves. The vascular bundles are surrounded by a layer of thick walled bundle sheath cells. Look at Figure 6.5 (a) and find the position of xylem in the vascular bundle.
6.2.6 Isobilateral (Monocotyledonous) Leaf
similar in appearance on both the sides
The anatomy of isobilateral leaf is similar to that of the dorsiventral leaf in many ways. It shows the following characteristic differences.
In an isobilateral leaf, the stomata are present on both the surfaces of the epidermis; and the mesophyll is not differentiated into palisade and spongy parenchyma (Figure 6.5 b).
In grasses, certain adaxial epidermal cells along the veins modify themselves into large, empty, colourless cells called bulliform cells.
The parallel venation in monocot leaves is reflected in the near similar sizes of vascular bundles (except in main veins) as seen in vertical sections of the leaves.
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