INTERNAL STRUCTURES OF ROOTS, STEM AND LEAVES

Learning Objectives

After studying or reading this, you should be able to:

1. Describe the internal structures of roots, stem and leaf.

2. Distinguish between different types of tissues of the root, stem and leaf.

3. Relate the structure of the types of cells to their functions.

4. Compare transverse section of root and stem of monocot and dicot plants.














The stems and roots of vascular plants differ in structure, but grow their apices and consist of the same three kinds of tissues. Epidermis covers their external surfaces, vascular tissue conducts materials within them, and ground tissue performs photosynthesis and stores nutrients. One component of the vascular tissue, xylem, conducts water and its dissolved minerals; and the other component, the phloem, distribute carbohydrates, which are manufactured in the green parts of the plant.


THE STEM
   The transverse section of herbaceous stem consists of four main tissues. These are epidermis, cortex, vascular bundle {Tissues} and pith.





THE EPIDERMIS
     This is a tissue of a single layer of closely fitting cells, which forms a continuous layer around the stem. The outer walls are impregnated with waxy materials forming a cuticle. A number of types of specialized cells occur in the epidermis, including guard cells, trichomes, and root hairs. The guard cells are paired, and the pairs flank stomata. The stomata are the epidermal opening through which gas exchange takes place and water is lost. Stomata occur in the epidermis of leaves, and they sometimes occur on other parts of the shoot, such as stems or fruits. The passage of carbon dioxide and oxygen into and out of the leaves, as well as the loss of water from them, takes place almost exclusively through the stomata. In most kinds of plants, stomata are more numerous on the lower surface of the leaf than on its upper surface, and in many plants they are entirely confined to the lower surface. The stomata open and shut in relation to external factors such as the supply of moisture. During period of active photosynthesis, the stomata are open, allowing the free passage of carbon dioxide into the leaf.

Trichomes are outgrth of the epidermis that varies greatly in form in different kinds of plants. They occur frequently on stems, leaves and reproductive organs. Functionally, the epidermis (cuticle) protects the shoot against water loss, mechanical damage and the entry of harmful bacteria and fungi.


THE CORTEX
   The cortex lies below the epidermis. it consists of large parenchyma cell within walls and narrow air spaces between them. The air space forms a continuous system, which permits gases to difuse to and from the surface and all living cells of the stem. The turgidity of parenchyma cells gives support to the stem. Another group of cells that gives additional support to some plants are collenchyma. These are found in the cortex beneath the epidermis and consist of small cells, thickened at the corners and without air spaces between them.

The innermost layer of the cortex is the endodermis. it has a single layer of rectangular shaped cells which are closely packed without intercellular spaces. it contains abundance starch grains. Endodermls forms ring around the central (vascular bundles).

Pericycle lies below the endodermis and the vascular bundles. It is made up of sclerenchyma cells which give strength and rigidity to the plant.


VASCULAR BUNDLES
    This comprises phloem on the outside and xylem inside. The phloem is separated from the xylem by the cambium layer. The phloem conducts food to all parts of the plants whilst the xylem conducts water and dissolved substances from the root to leaves.

Two different kinds of elongated, slender conducting cells occur in the phloem: sieve cells and sieve plates members. These cells are living but neither of them has nucleus. In sievetube members, the pores in some of the sieve areas are larger than those of others. Such sieve areas are called sieve plates. Sieve tube members occur end to end, forming longitudinal series called sieve tubes.

Special parenchyma cells called companion cells occur regularly in association with the sieve-tubes. Companion cells carry out some of the metabolic functions that are needed to maintain the individual sieve-tube members with which they are associated.

The xylem is the special water-conducting tissue of plants. It is a woody tissue found in vascular plants and it conducts water and mineral salts throughout the plant and provides it with mechanical support. In leaves, flowers, and young stems, xylem is present in conjunction with phloem in the form of conducting strands called vascular bundles. ln roots there is a central core of xylem. Xylem that derives from the shootand root-growing points is called primary xylem. in addition, new xylem, called secondaryxylem, may be added by division of the cambium cells which is located between the xylem and phloem. This division gives rise to new xylem cells towards the centre in roots and towards the outside in most stems. Some plants, however, have little or no secondary xylem.





Xylem may contain three types of elongated cells: tracheids, vessel elements, and fibres. At maturity, when functioning as transport, all of these cells are dead. Tracheids are elongated cells with thick walls characterized by small, sharply defined thin areas known as pits. Vessel elements are specialized tracheids in which the end walls have one or more pores. Fibres are specialized tracheids with muchthickened walls. They function only slightly in transport, but serve to increase the strength of xylem. In most angiosperms (flowering plants) the xylem contains well-developed vessels and fibres. Cambium layer is located between the xylem and the phloem. It contains actively dividing cells. The division of the cambium cells increases the size and width of the xylem and phloem vessels.




PITH
   It occupies the whole centre of the stem. It extends outwards in between the bundles up to the pericycle. Each extension of the pith forms the medullary rays.

PRIMARY TISSUE 0F MONOCOT STEM The monocot stem has similar feature as the Dicot except the following: There is no vascular cambium, no seconding thickening due to absence of vascular cambium. The vascular bundles are scattered throughout the ground parenchyma, no distinct cortex and pith, and each vascular bundle is surrounded by a layer of sclerenchyma.







INTERNAL STRUCTURE OF ROOT Internally, a root has the following parts: epidermis, cortex, vascular tissue (xylem & phloem) and root cap.


TRANSVERSE SECTION SHOWING INTERNAL STRUCTURES OF DICOTYLEDONOUS ROOT


These structures are put into four main zones/regions including .. the root cap, meristematic region, region of elongated and region of maturation (or differentiation).



The Root Cap (Region of Cell Division)
    One of its functions is to protect the delicate tissues of the root from mechanical damage as the young root grows through the soil. The region is composed of a loosely arranged parenchyma cells located at the tip of the roots. The root tip pushes through abrasive soilparticles. The parenchyma cells, which are re laced from the inside, constantly; slough off, forming a slimy lubricant that facilitates movement through the soil.


MERISTEMATIC REGION
   Cells in this region produce the root cap. It is composed of apical meristem i.e. a region of active cell division that occurs relatively close to the tips of roots and shoot of plants. This adds new cells to the root cap and also provides new cells that increase the length of the root.


REGION OF ELONGATION
    Cells produce in the meristematic region absorb water and develop vacuoles. As water fills the vacuole the cells elongates. The increase in the length of these cells pushes the meristematic region and the root cap further into the soil.


REGION OF MATURATION
In this region most of the cells mature into the various distinctive cells type of the primary tissues. This region is often referred to as the region of differentiation. Primary tissue formed includes the epidermis, vascular tissue (xylem & phloem) and root hairs. The root hairs often grow from an area of the epidermis called the piliferous layer. These root hairs form part of the epidermis cells and greatly increase the absorptive surface of the root as they absorb water and mineral ions from the soil.





INTERNAL STRUCTURE OF A LEAF
A typical leaf consists of three main regions:
These are: epidermis mesophyll and vascular bundle.






EPIDERMIS
   This is a single layer of cells occurring on both surface of the lamina/leaf blade. It consists of cells which have no chloroplast. The epidermis on the lower surface of the lamina can be distinguished from that of the upper surface by the presence of the stomata (minute pore). Two beans-shaped cells called guard cells enclosed each stoma. These cells contain chloroplasts with which they regulate the size of the stomata by varying their turgidity and thus control gas exchange between internal environment of the leave and the atmosphere.


CUTICLE
The exposed surface of the cells of the upper and lower epidermis is covered with cuticle. Functionally, the cuticle prevents loss of water from the leaf thorough the epidermal cells. It also protects the internal tissues and cells from mechanical damage by bacteria and fungi.


STOMATA
   These are tiny holes or openings in the epidermis.
They are found between epidermal cells.
They vary in size in different plants. Stomata are more numerous in the lower epidermis than in the upper epidermis. Functionally. the stomata allow entry and exits of carbon dioxide and oxygen which are used for photosynthesis and respiration respectively.


PALISADE MESOPHYLL
    Located immediately below the upper epidermis. The cells contain many chloroplasts. Because it contains many chloroplasts, its main function is to carry out photosynthesis.


MESOPHYLL
    This consists of the tissue between the upper and lower epidermis. its cells are thin-walled and contain chloroplasts. Unlike the monocot, the mesophyll of the dicot is made up of two types of cells. The upper mesophyll cell has many chloroplasts and spaces between them and this constitute the palisade tissue. The latter is made up of irregular shaped; loosely packed cells with fewer chloroplasts and air spaces between them make up the spongy tissue, within these two layers, is the vascular bundle.


THE VASCULAR BUNDLE
    Veins of various sizes are scattered throughout the mesophyll. They consist mainly of xylem and phloem tissues, and provide mechanical strength, i.e. provide its skeleton. The carbohydrates produce in the mesophyll cells are transported in solution throughout the plants by the phloem whilst the xylem conducts Water and mineral ions into the leaf.
















Have you UNDERSTOOD what you have just studied or read?

To make sure you have UNDERSTOOD, ANSWER the QUESTIONS below:

INTERNAL STRUCTURE OF ROOTS, STEMS AND LEAVES

1. a. What is atissue?

b. Describe a named plant tissue that serves for transport.

2. Draw and label a diagram of the transverse section of a dicotyledonous leaf as seen under the low power of the microscope.

3. a. Describe the structure of the epidermis of a dicotyledonous leaf as seen under the high power of the microscope.

b. Relate the structure of the epidermis to its function.

4. a. Name the four primary tissues of the stem of a dicotyledonous plant.

b. Describe the structure of each of the tissues you have named in (a) above as seen in transverse section.

5. a. Name four types of supporting tissues in flowing plants.

b. State four general functions of the supporting tissues named in (a) above.

6. Make a large drawing 8-10cm long of the transverse section of a dicotyledonous stem and label fully.

7. Explain how a flowering plant obtains
i. Water ii. Mineral salts

8. Describe the pathway and mechanism of water transport in the plant.

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