UNIT 1. PARTS AND FUNCTIONS OF A PLANT

THE PLANT

A plant is a living organism. It is made up of different parts, each or which has a particular purpose, or specialized function.

If one part of the plant is not functioning properly the whole plant will suffer. But we may cut flowers off the plant or prune the roots or the branches. Such damage is only temporary and so the plant will continue to grow.

Plants have two Organ Systems: the shoot system and the root system.

1. The root system is below ground. This system includes roots, tubers (an enlarged, fleshy, underground stem with buds capable of producing new plants) and rhizomes (a horizontal stem that is usually underground and roots grow from it.)

2. The shoot system is above ground. This system includes these organs: stems, leaves, flowers, fruits and seeds. 


THE ROOT

The root is usually below the ground. It has two main functions:

1.-It absorbs the water and minerals from the soil and sometimes stores food.

2.-It holds or anchors the plant firmly in the soil.

The primary roots and its branches constitute the tap root system. Tap roots of carrot, turnip and adventitious roots of sweet potato get swollen and store food.

In monocotyledonous plants, the primary root is short lived and is replaced by a large number of roots. These roots originate from the base of the stem and constitute the fibrous root system, as seen in the wheat plant.

 

In some plants, like grass, Monstera and the banyan tree, roots arise from parts of the plant other than the radicle and are called adventitious roots.

Hanging structures that support a banyan tree are called prop roots. Similarly, the stems of maize and sugarcane have supporting roots coming out of the lower nodes of the stem. These are called stilt roots.

THE STEM

A stem is one of two main structural axes of a plant, the other being the root.

The stem is normally divided into nodes and internodes. The region of the stem where leaves are born are called nodes while internodes are the portions between two nodes.

One of the functions of the plant is to support the plan.

Another important function of the stem is to enable water and minerals to pass up from the root to the leaves and flowers.

Some stems perform the function of storage of food.

Axillary buds of stems may also get into woody,straight and pointed thorns. The thorn is technically a modified, sharp-pointed stem. Thorns are found in many plants such as Citrus, Bougainvillea. They protect plants from browsing animals.

Some plants of arid regions modify their stems into flattened or fleshy cylindrical structures. They contain chlorophyll and carry out photosynthesis.

The leaves grow out of the side of the stem. Their main job is to make food for the plant by the process known as photosynthesis. For this process sunlight is necessary. Water from the soil and carbon dioxide from the air are converted into sugar and other carbohydrates. During the process oxygen is formed and released into the air. 

A leaf develops at the node and bears a bud in its axil. The axillary bud later develops into a branch.

A typical leaf consists of three main parts: leaf base, petiole and lamina.

The petiole helps hold the blade to light. Long thin flexible petioles allow leaf blades to flutter in the wind, thereby cooling the leaf and bringing fresh air to the leaf surface.

The lamina or the leaf blade is the green expanded part of the leaf with veins and veinlets. There is, usually, a middle prominent vein, which is known as the midrib. Veins provide rigidity to the leaf blade and act as channels of transport for water, minerals and food materials.

Leaves are often modified to perform functions other than photosynthesis. They are converted into tendrils for climbing as in peas or into spines [thorns] for defense as in cacti. The fleshy leaves of onion and garlic store food.

Root Modification	Stem Modification	Leaf Modification
Carrot and Sweet Potato.	Potato and Ginger	Onion and Garlic.

 

  THE FLOWER

The flower contains the reproductive organs of the plant.

The stamens produce the male sex cells, or spermatia which are carried in the pollen grains.

The carpel produces the female sex cells, or ovules.

Androecium is composed of stamens. Each stamen which represents the male reproductive organ consists of a stalk or a filament and an anther. The pollen grains are produced in pollen-sacs. A sterile or infertile stamen is called staminode.

Gynoecium is the female reproductive part of the flower and is made up of one or more carpels. A carpel consists of three parts namely stigma, style and ovary.

After fertilization, the ovules develop into seeds and the ovary matures into a fruit.

When a flower has both androecium and gynoecium, it is bisexual. A flower having either only stamens or only carpels is unisexual.

 THE FRUIT

 The fruit is the ripened ovary of the flower. It usually encloses the seeds and protects them while they are developing.

The fruit is usually fleshy and sweet smelling.

The pericarp is the tissues of a fruit surrounding the seed(s). Pericarps consist of three layers (although not all fruits have all layers): endocarp, mesocarp, and exocarp.


THE SEED

A seed is a fertilized ovule containing the plant embryo. Given the appropriate growth conditions, it will become the new plant.

Plants such as ferns, mosses and liverworts do not produce seeds.

Certain seeds are edible (e.g. legumes, nuts, and cereals). Edible seeds are an essential component of an animal diet due to its high content of concentrated nutrients, e.g. starches, proteins, and fats. Other seeds such as sunflowers, rapeseed, and cottonseed are used commercially in the production of oils (fats).