Biology – Gurukula JL/DL SPECIAL | Connection.. Coordination.. Circulation
ANATOMY OF FLOWERING PLANTS THE TISSUE SYSTEM
- On the basis of their structure and location, there are three types of tissue systems. These are the epidermal tissue system, the ground or fundamental tissue system and the vascular or conducting tissue system.
Epidermal Tissue System
- The epidermal tissue system forms the outer-most cove ring of the whole plant body and comprises epidermal cells, stomata and the epidermal appendages – the trichomes and hairs. The epidermis is the outermost layer of the primary plant body. It is made up of elongated, compactly arran ged cells, which form a continuous layer. Epidermis is usually singlelayered.
- Epidermal cells are parenchy matous 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 and gaseous exchange. Each stoma is composed of two beanshaped cells known as guard cells which enclose stomatal pore. In grasses, the guard cells are dumb-bell shaped. The outer walls of guard cells (away from the stomatal pore) are thin and the inner walls (towards the stomatal pore) are highly thic kened. The guard cells possess chloroplasts and regulate the opening and closing of stomata. 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 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.
The Ground Tissue System
- All tissues except epidermis and vascular bundles constitute the ground tissue. It consists of simple tissues such as parenchyma, collenc hyma 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.
The Vascular Tissue System
- The vascular system consists of complex tissues, the phloem and the xylem.The xylem and phloem together constitute vascular bundles. In dicotyledonous stems, cambium is present bet ween phloem and xylem. Such vascular bundles because of the presence of cambium possess the ability to form secondary xylem and phloem tissues, and hence are called open vascular bundles. 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. When xylem and phloem within a vascular bundle are arranged in an alternate manner along the different radii, the arrange ment is called radial 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 vascular bundles are common in stems and leaves. The conjoint vascular bundles usually have the phloem located only on the outer side of xylem.
ANATOMYOF DICOTYLEDONOUS AND MONOCOTYLEDONOUS PLANTS
- For a better understanding of tissue organisation of roots, stems and leaves, it is convenient to study the transverse sections of the mature zones of these organs.
Dicotyledonous Root - it shows the transverse section of the sunflower root. The internal tissue organisation is as follows
- The outermost layer is epiblema. Many of the cells of epiblema protrude in the form of unicellular root hairs. The cortex consists of several layers of thin-walled paren chyma cells with intercellular spaces. The innermost layer of the cortex is called endodermis.
- It comprises a single layer of barrelshaped cells without any intercellular spaces. 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. Next to endodermis lies a few layers of thick-walled parenchyo matous cells referred to as pericycle. Initiation of lateral roots and vascular cambium during the secondary growth takes place in these cells. The pith is small or incons picuous.
- The parenchymatous cells which lie between the xylem and the phloem are called conjuctive tissue. There are usually two to four xylem and phloem patches. Later, a cambium ring develops between the xylem and phloem. All tissues on the innerside of the endodermis such as pericycle, vascular bundles and pith constitute the stele.
Monocotyledonous Root - The anatomy of the monocot root is similar to the dicot root in many respects It has epidermis, cortex, endode rmis, pericycle, vascular bundles and pith. 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.
Dicotyledonous Stem - The transverse section of a typical young dicotyledonous stem shows that the epidermis is the outermost protective layer of the stem Root hair Covered with a thin layer of cuticle, it may bear trichomes and a few stomata. The cells arranged in multiple layers between epidermis and pericycle constitute the cortex. It consists of three sub-zones. The outer hypodermis, consists of a few layers of collenchymatous cells just below the epidermis, which provide mechanical strength to the young stem. Cortical layers below hypodermis consist of rounded thin walled parenchymatous cells with conspicuous intercellular spaces. The innermost layer of the cortex is called the endodermis. 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. A large number of rounded, parenchymatous cells with large intercellular spaces which occupy the central portion of the stem constitute the pith.
Monocotyledonous Stem - The monocot stem has a sclerenchyma tous ypoder mis, a large number of scattered vascular bundles, each surrounded by a sclerenchymatous bundle sheath, and a large, conspicuous parenchymatous ground tissue Vascular bundles are conjoint and closed. 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.
