The root system of a flowering plant begins its development from the hypocotyl of the embryo of the seed which gives rise to the primary root. Roots generally grow downwards into the soil (positively geotropic) and upwards (negatively geotropic). Roots do not bear leaves and therefore no nodes are present.Two kinds of root systems can be distinguished in flowering plants: tap root systems and adventitious root systems. Usually dicotyledons posses tap root systems and monocotyledons adventitious root systems.

Tap Root System: The primary root grows vertically down into the soil in the tap root system. Later lateral or secondary roots grow from this at an acute angle outwards and downwards, and from these other branches may arise. The main or primary root is known as the tap root; together with its many branch roots it forms a tap root system e.g. the bean.

Adventitious Root System: The primary root usually dies at an early stage and is replaced by numerous roots that develop from the stem. These roots, which develop from the stem, are equal in size. They are known as a adventitious roots, which give rise to branch or lateral roots and form an adventitious root system, e.g. the mealie.

Functions of the Root

Roots anchor the plant in the soil.

Roots absorb water and mineral salts from the soil.

Roots may store food.

Roots form a passage way for water and dissolved substances from the root into the stem and also for foods from the stem down into the root.

Root of a Dicotyledonous Plant: External Features

If we study the root tip of a flower plant under a dissecting microscope or with a hand lens, the following regions can be distinguished:

The Root Cap

The tip of the root is covered by a cap that is shaped like a thimble. The outer cells of the root cap are continuously being worn away and new cells are added to the inner portion. As these cells disintegrate they form a strong protective cover.

Function

The root cap covers and protects the delicate growing tip from injury and damage as the root pushes its way through the soil.

The Meristematic Region or Growing Point

This region occurs immediately behind the root cap. The Meristematic region consists of meristematic tissue and is protected by the root cap. The cells are very active and divide rapidly to form new cells which differentiate later to form more specialized root tissues. The cells of this region also replace the cells rubbed-off from the root cap and provide additional cells for the next region.

Function

This is the region where the cells divide and where additional cells are provided for the zone of elongation.

The Region of Elongation

The cells formed in the meristematic region undergo rapid growth in length. This causes the root to elongate and penetrate deeper into the soil in search for water and mineral salts. The meristematic and elongation zones are also referred to as the region of growth.

Function

In this region the cells undergo rapid enlargement,i.e. the cells undergo rapid growth in length.

The Root-hair Region

The root hair region occurs a short distance above the region of elongation. Here a large number of fine, hair-like outgrowths are formed. The root hairs arise from the epidermal tissue of the root and are called root hairs. The root hairs are short and short-lived and develop on the primary and secondary roots. A root-hair consists of the following parts: a thin cell wall, a thin lining of cytoplasm which contains the nucleus and a comparatively large vacuole containing cell sap.

Function

The main function of the root hairs is to increase the area of absorption of the root.

The Mature Region

The mature region is situated above the root hair region. Here the root becomes thicker and secondary or lateral roots are developed. The secondary roots in turn rebranch to form tertiary roots. Each lateral branch has its own cap, root hairs, meristematic, elongation and mature regions. The roots in this region are covered by a protective cork layer .

Functions

In this region the cells undergo differentiation into specialized cells,(e.g. xylem and phloem).

It also serves to anchor the plant.

In vascular plants, the root is the organ of a plant body that typically lies below the surface of the soil. But, this is not always the case, since a root can also be aerial (that is, growing above the ground) or aerating (that is, growing up above the ground or especially above water). On the other hand, a stem normally occurring below ground is not exceptional either (see rhizome). So, it is better to define root as a part of a plant body that bears no leaves, and therefore also lacks nodes. There are also important internal structural differences between stems and roots. The two major functions of roots are 1.) absorption of water and inorganic nutrients and 2.) anchoring the plant body to the ground. Roots also function in cytokinin synthesis, which supplies some of the shoot's needs. They often function in storage of food. The roots of most vascular plant species enter into symbiosis with certain fungi to form mycorrhizas, and a large range of other organisms including bacteria also closely associate with roots.

At the tip of every growing root is a conical covering of tissue called the root cap. It usually is not visible to the naked eye. It consists of undifferentiated soft tissue (parenchyma) with unthickened walls covering the apical meristem. The root cap provides mechanical protection to the meristem cells as the root advances through the soil. Its cells are worn away, however, they are quickly replaced by new cells generated by cell division within the meristem. The root cap is also involved in the production of mucigel, a sticky mucilage that coats the new formed cells. These cells contain statoliths, starch grains that move in response to gravity and thus control root orientation.

Stem

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A stem is one of two main structural axes of a vascular plant. The stem is normally divided into nodes and internodes, the nodes hold buds which grow into one or more leaves, inflorescence (flowers), cones or other stems etc. The internodes act as spaces that distance one node from another. The term shoots is often confused with stems; shoots generally refer to new fresh plant growth and does include stems but also to other structures like leaves or flowers. The other main structural axis of plants is the root. In most plants stems are located above the soil surface but some plants have underground stems.

Stems have four main functions which are:[1]

Support for and the elevation of leaves, flowers and fruits. The stems keep the leaves in the light and provide a place for the plant to keep its flowers and fruits.

Transport of fluids between the roots and the shoots in the xylem and phloem.

Storage of nutrients.

The production of new living tissue. The normal life span of plant cells is one to three years. Stems have cells called meristems that annually generate new living tissue.

Structure

Stem usually consist of three tissues, dermal tissue, ground tissue and vascular tissue. The dermal tissue covers the outer surface of the stem and usually functions to waterproof, protect and control gas exchange. The ground tissue usually consists mainly of parenchyma cells and fills in around the vascular tissue. It sometimes functions in photosynthesis. Vascular tissue provides long distance transport and structural support. Most or all ground tissue may be lost in woody stems. The dermal tissue of aquatic plants stems may lack the waterproofing found in aerial stems. The arrangement of the vascular tissues varies widely among plant species.

Leaves

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A structurally complete leaf of an angiosperm consists of a petiole (leaf stem), a lamina (leaf blade), and stipules (small processes located to either side of the base of the petiole). The petiole attaches to the stem at a point called the "leaf axil". Not every species produces leaves with all of the aforementioned structural components. In some species, paired stipules are not obvious or are absent altogether. A petiole may be absent, or the blade may not be laminar (flattened). The tremendous variety shown in leaf structure (anatomy) from species to species is presented in detail below under Leaf morphology. After a period of time (i.e. seasonally, during the autumn), deciduous trees shed their leaves. These leaves then decompose into the soil.

A leaf is considered a plant organ and typically consists of the following tissues:

An epidermis that covers the upper and lower surfaces

An interior chlorenchyma called the mesophyll

An arrangement of veins (the vascular tissue).

In botany, a leaf is an above-ground plant organ specialized for photosynthesis. For this purpose, a leaf is typically flat (laminar) and thin, to expose the cells containing chloroplast (chlorenchyma tissue, a type of parenchyma) to light over a broad area, and to allow light to penetrate fully into the tissues. Leaves are also the sites in most plants where transpiration and guttation take place. Leaves can store food and water, and are modified in some plants for other purposes. The comparable structures of ferns are correctly referred to as fronds. Furthermore, leaves are prominent in the human diet as leaf vegetables.

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