Ncert Questions
Question 1. State the location and function of different types of Meristems.
Answer: (i) Apical meristems: o Location:These meristems occur at the tip of the roots and the tip of the shoots the shoot apical meristems mostly occupy the distant most region of the stem axis. o Function:They help in the elongation of the root and shoot. As they appear in the early life of the plant they contribute to the formation of the primary plant body. (ii) Intercalary meristems: o Location:These meristems occur between mature tissues. They are also present in the axils of the leaf. o Function:They are capable of forming branches and flowers. In grasses they help in regenerating the parts removed by the grazing animals. Both the above meristems are called primary meristems. (iii) Secondary or Lateral meristems: o Location:They occur in the mature regions of roots and shoots especially in the woody axis. o Function:Their main function is to produce the secondary tissue.
Question 2. Cork cambium forms tissues that form the cork. Do you agree with this statement? Explain.
Answer: Phellogen or cork cambium is a couple of layers thick. These layers are made of narrow, thin-walled and nearly rectangular cells. Cork cambium cuts off cells on both sides of these the outer cells differentiate into cork or phellem while the inner cells differentiate into secondary cortex or phelloderm. The Phellem and Phelloderm formed from the cork cambium or Phellogen are collectively called periderm.
Question 3. Cut a transverse section of young stem of a plant from your school garden and observe it under the microscope. How would you ascertain whether it is a monocot stem or a dicot stem? Give reasons.
Answer: If the stem is a monocot stem it will show the following characters: Internal structure of a monocotyledons stem: Epidermis:This is the outermost layer of the stem, which is made of a single layer of cells. The epidermis does not bear unicellular or multicellular hairs. Hypodermis:This is just below the epidermis it is made up of sclerenchymatous cells. It gives mechanical strength to the stem. Cortex:It is made up of conspicuous parenchymatous ground tissue. Vascular bundles: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. If the stem is dicot stem it will show the following characters: Internal structure of a dicotyledonous stem:Epidermis:This is the outermost layer of the stem. It consists of a single layer of cells. The epidermis bears multicellular, uniseriate trichomes. A thin layer of cuticle is present on the epidermis as well as the tichomes.Cortex:The cortex consists of several layers of cells. This is divided into three sub-zones, namely hypodermis, general cortex and endodermis. Hypodermis:This is just below the epidermis and consists of 3 to 4 layers of collenchymatous cells. It gives mechanical strength to the stem. These cells are thickened at the corners and possess chloroplasts.General cortex:The cells of this zone are parenchymatous and multi-layered. Oil ducts, surrounded by a glandular parenchymatous layer, also occur copiously throughout this sub-zone.Endodermis:It is the innermost layer of the cortex and consists of a layer of barrel-shaped cells. Since it is rich in starch, it is also referred to as the starch sheath.Pericycle:It is in the form of semi lunar patches of sclerenchyma. Each patch associated with phloem of the vascular bundle, is called the hard bast.Medullary rays:In between the vascular bundles there are a few layers of parenchymatous cells, which constitute the medullary rays. These are slightly larger in size when compared to the other cortical cells. Usually, these are polygonal in shape and exhibit no intercellular spaces in between.Vascular bundles:The vascular bundles are arranged in a ring internal to the endodermis. Each vascular bundle is conjoint, collateral, endarch and open. It is composed of xylem, phloem and cambium.Phloem:This is situated on the outer side of the vascular bundle. The cells are thin-walled and polygonal. Phloem is a complex tissue and is composed of sieve tube elements, companion cells, the phloem parenchyma and phloem fibres.Xylem:The xylem tissue lies below the phloem. This is composed of vessels, tracheids, xylem parenchyma and fibres.Cambium:This is present in between the xylem and the phloem. It consists of 2-3 layers of thin-walled, rectangular cells.Pith:This is the central portion of the stem. It consists of rounded, parenchymatous cells with plenty of intercellular spaces.