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A flowering plant is the type of plants which has got roots, stems, leavesand whosereproductive structuresare flowers. The roots of a plant are usually found underground. All the parts of vascular plants have an outer covering of protective tissue and an inner matrix of tissue within its embedded vascular tissue that conducts water, nutrients, and food throughout the plant.

A vascular plant is organized along a vertical axis, like a pipe. The part below ground is calledthe root; the part above ground is called theshoot. The root penetrates the soil and absorbs water and various ions, which are important for plant nutrition.

Roots

The functions of roots are:

Hold the plant in the soil and take in water and minerals from the soil. Water and minerals are carried upward to the stem and transport dissolved food downwards from the stem. In addition, the roots of some plants are specialised for storage of food.

The first structure to emerge from a sprouting seed is the primary root. As the plant grows and matures secondary roots develop which are affected by obstructions and other factors such as moisture and the chemical makeup of the soil. This results in irregular ways of root growth with frequent bends.

The two common systems of roots are the taproot and fibrous roots. When the primary root grows rapidly and remains the largest root in the root system, it is called a taproot system. Taproot systems grow deep into the soil and grow thick and fresh as in carrots, and oak trees.

A fibrous root system is made up of numerous roots, many of which are nearly the same in size as in maize and grasses. Sweet potatoes are fibrous root systems while carrot and beetroots are modified storage taproot. Adventitious root system grows from stems or leaves.

Root growth, internal structure and functions

The core of the root is the central cylinder or vascular cylinder that is surrounded by a layer of parenchyma cells called pericyclewhich is inside the endodermis. From the pericycle layer originate all secondary roots. Secondary growth in roots is similar to stems where annual growth rings are formed. At the centre of the vascular cylinder is where the conducting tissues-xylemandphloemare located. The pericyclic produces the cork cambium.

Though the branches of a root system may be many metres long, only a small region at the tip gradually grows extending into the soil. The root tip is made up of several different zones, each zone with cells at different stages of development. The main ones in successive zones are root cap, meristematic zone, elongation zone, and maturation zone.

The root cap is a thimble-shaped group of cells that form a protective covering for the delicate meristematic tissues of the root tip behind it. The outer cells of the root cap get crushed as the cells behind the root tip push it through the soil. The crushed cells release a fluid that enables the root tip through the soil. New root cap cells are continuously formed by the meristematic tissue.

Behind the root cap is a region of actively dividing cells called the meristematic region. The cells of this region are small and thin-walled. This is where all the cells of the root are formed.

The‘elongation zone’comes after the meristematic zone. The cells that were initially made in the meristematic zone enlarge and push the root tip forward. Behind the elongation zone is thematuration zoneor root hair zone where the cell differentiate. In both the root and the stem, cells develop into fully grown, functioning ceils of various types, such as xylem, phloem, and parenchyma.

A cross section through the maturation zone will show that the root is made up of several tissue layers. The epidermis is the outer most layers that are responsible in the taking in of water and minerals from the soil. Root hairs develop from the epidermal cells. These root hairs provide a large surface area for absorption of water. The cortex is just behind the epidermis. The parenchyma cells of the cortex store the plant’s food, which is mainly starch. The innermost layer of the cortex is called the endodermis and its main function is to control water into the central cylinder.

In the roots of woody plants, a vascular cambium develops between the xylem and phloem. The vascular cambium adds new xylem to its inside and new phloem to its outside.

Types of stems

There are basically twotypes of stems– herbaceous stems and woody stems. Herbaceous plants have soft, green, juicy stems that are called herbaceous stems. They have a life span of one to two years. The cells of apical meristems are the source of all the tissues of herbaceous plants. Herbaceous stems are produced by primary growth. Examples of herbaceous dicot include sunflower, buttercup, and alfalfa.

The stems of a plant have several functions. Vascular tissue runs through the stem, transporting water, food, and minerals between the roots and the leaves. Some underground stems, such as the white potato tuber, are specialized for food storage.

The cactus stems are modified for storage of water and photosynthesis. The stems of strawberry plants have stems running along the surface of the ground and develop independent plants.

The shoot consists of stem and leaves. The stem serves as a framework for the positioning of the leaves, where most photosynthesis takes place. The arrangement, size, and other characteristics of the leaves are very important in the production of food in plants. Flowers, fruits and seeds are also formed on the shoot.

1.Internal structure of woody stems

Woody plants have woody stems that are made up of thick, tough tissue that you know as wood. There life span is usually more than two years.

All woody plants are dicots. The stems are tough because of the large amount of xylem that are kept on being added to the thickness of the stem. As shown below, round layers of wood increase the thickness of the stem as xylem builds on the inside of the vascular cambium. The phloem produced by the vascular cambium, does not build up. What happens is that its older layers break off and become new phloem.

1.Internal structure of herbaceous stems

The outermost tissue is the epidermis and is covered with waxy cuticle to prevent water loss. The vascular tissue is found in bundles that are arranged in a ring (dicots) or scattered (monocots). The central region of the stem is called the pith. Pith is made ofparenchyma cellsthat store food.

The growth of new xylem during each growing season results in the formation of annual rings as shown below. Counting the rings can determine the age of a dicot plant. Growth each year is represented by an annual ring.

In young woody dicots, the centre of the stem is filled with pith. The cortex layer is inside the epidermis. For older woody stems, the cells of the pith die, and the cortex is replaced by phloem from the vascular cambium. The xylem lies next to the cambium.

The protective tissue, the bark, is the outermost layer of a woody stem. Bark is made of phloem, cork cambium and cork cells. The cork cells are made by the cork cambium. The inner, younger part of the bark is alive, but outer older part is dead tissue. The older outer bark cracks and comes off as new bark develops.

2.External structure of woody stems

At the tip of the plant is the terminalbudwhich is made of apical meristems enclosed by overlapping protective scales calledbud scales. Within the leaf scars are dots calledvascular bundle scars. These are the areas at which vascular bundles consisting of xylem and phloem passed from the stem into the leaf.

Theaxillaryorlateral budis found each leaf scar. These are found above the area where a leaf is or was attached to the stem. These buds may grow into new branches, or may remain small and dormant. Nodes are points along the stem where leaves and literal buds form. The space between two nodes is called aninternode. Lenticels are holes that pass through the cork tissue which allows the exchange of oxygen and carbon dioxide between the atmosphere and the internal tissues.

Transpiration is the evaporation of water from plants. It occurs basically at the plant’s leaves while theirstomataare open for the passage ofcarbon dioxideand oxygen during the chemical process ofphotosynthesis. The diagram of a plant below shows the movement of water from the soil into the leaves up to the leaves.

This transpired water must be replaced by the transport of more water from the soil to the leaves through the xylem vessel of the roots and stem.

1.Importance of transpiration

Transpiration is the process that pulls water up from the roots to:

1) supply water for photosynthesis,

2) bring minerals from the roots for biosynthesis within the leaf, and

3) Cool the leaf.

2.Translocation

The phloem conducts carbohydrates (mostly sucrose) to where they are needed by the plant as food. Food conduction in phloem is carried out through two kinds of elongated cells,the sieve cellsandsieve tube members. The movement of sugars and other substances from one region to another through the sieve cells is called translocation:

3.Factors affecting transpiration

Investigating the factors affecting the rate of transpiration

Using a potometer (right), one can study the effect of various environmental factors on the rate of transpiration. As water is transpired or otherwise used by the plant, it is replaced from the reservoir on the right. This pushes the air bubble to the left providing a precise measure of the volume of water used.

4.Environmental factors that affect the rate of transpiration

(i)Light

Plants transpire more rapidly in the light than in the dark. This is largely because light affects the opening of the stomata. Stomata open fully when a plant receives more light than when it is dark.

(ii)Temperature

Green plants transpire faster at higher temperatures because water evaporates more rapidly as the temperature rises. For example, at 30°C, a leaf may transpire three times as fast as it does at 20°C. High temperatures make more water evaporate from leaves.

1.Humidity

The rate of diffusion of any substance increases as the difference in concentration of the substances in the two regions increase. When the surrounding air is dry, diffusion of water out of the leaf goes on more quickly.

Wind

On windy days, transpiration increases as the wind blows around the leaves, blowing away any water vapour that diffused out from the leaf and could be on the surfaces When the air is still, the air surrounding a leaf becomes increasingly humid thus reducing the rate of transpiration.

Soil water

A plant cannot continue to transpire rapidly if its water loss is not by replaced from the soil. When absorption of water by the roots fail to keep up with the rate of transpiration, loss of turgor occurs, and the stomata close. This immediately reduces the rate of transpiration (as well as of photosynthesis). If the loss of turgor extends to the rest of the leaf and stem, the plant wilts.

5.Control of transpiration in plants

Plants in areas where rain is seasonal have specially developed features to overcome excessive loss of water by transpiration. We say they have adaptations for conserving water. The following are some ways of adaptations for plants:

(iii) They shed their leaves in the dry season to survive through the dry season.

(iv) Some plants have leaves whose surfaces are hairy or leaves with sunken stomata for reduction in transpiration.

(v) Some trees such as mangoes, oranges, avocado and pine are evergreen. These retain their leaves all year round. The reduction in transpiration is because these leaves are covered with a layer of wax.

(vi) Other plants such as pine and cactus have small and needle-like leaves to ‘allow very little transpiration.

(vii) Fewer stomata on the upper surface of leaves like mango plants.

(viii) The upper surfaces are shinny, thus reflects some of the direct light.