Plant Systems
Complete Notes & Free Quiz

The root system is the underground part of a plant. It performs two essential functions.

First, it anchors the plant firmly in the soil so that it does not fall over or get blown away by the wind.
Second, it absorbs water and dissolved minerals from the soil and delivers them to the rest of the plant through the transport system.

A taproot is a single, large, thick root that grows straight down into the soil. It is the primary root from which smaller lateral roots branch off.

A carrot is a well-known example of a taproot. In a carrot, the taproot is also a food storage organ.

Other examples include radish, mustard and beetroot. These plants belong to the dicot group.

A fibrous root system has no single main root. Instead, it consists of many roots of similar size that spread out in all directions from the base of the stem.

Fibrous roots are shallow and cover a wide area of soil, which helps prevent soil erosion.

Examples include wheat, rice, maize and grass. These plants belong to the monocot group.

Adventitious roots arise from any part of the plant other than the primary root. They grow from stems or leaves instead of from the main root.

Types of adventitious roots:
Prop Root: Grows from lower nodes of the stem downward into the soil to provide extra support. Example: Banyan tree.
Stilt Root: Grows obliquely from lower stem nodes into the soil. Example: Maize, Sugarcane.
Pneumatophore: Grows upward from underground roots into the air to allow the plant to breathe in waterlogged conditions. Example: Mangrove.
Clinging Root: Grows from stem and helps the plant climb surfaces. Example: Money Plant.

The shoot system is made up of all the above-ground parts of a plant. It includes the stem, leaves, flowers, fruits and buds.

The shoot system performs three main roles: it carries out photosynthesis (mainly in the leaves), it provides support for the plant, and it carries out reproduction through flowers and fruits.

The stem is the main support structure of the plant above the ground. It performs several important functions:

It holds the leaves in a position where they can receive maximum sunlight for photosynthesis.
It holds the flowers upright so they can attract insects for pollination.
It acts as a transport highway, carrying water and minerals upward through xylem and food downward through phloem.
Some stems also store food and water, for example in cacti and sugarcane.

Leaves are the main food-making organs of the plant. Their primary function is photosynthesis, where they use sunlight, carbon dioxide and water to produce glucose and oxygen.

Leaves also carry out transpiration, releasing water vapour through stomata to help pull water upward from the roots.
They perform gas exchange, taking in carbon dioxide and releasing oxygen during the day.
Some leaves also store food and water, such as the leaves of aloe vera.

Buds are growing points of the plant. They contain meristematic tissue that divides and grows into new shoots, leaves and flowers. Buds are found at the tips of stems (apical buds) and in the axils of leaves (axillary buds).

Flowers contain the reproductive organs of the plant. Most flowers have brightly coloured petals and produce a sweet scent to attract insects for pollination, which leads to the production of seeds and fruits.

The transport system of a plant consists of two types of specialised tubes called xylem and phloem, which together form vascular bundles.

These vascular bundles run throughout the roots, stems and leaves of the plant, forming a continuous network that transports substances to every cell.

Xylem is a type of vascular tissue that carries water and dissolved minerals from the roots upward to all parts of the plant.

Xylem is made of dead cells joined end to end, forming continuous hollow tubes. The walls of xylem are thickened and strengthened with a substance called lignin, which also gives mechanical support to the plant. This thickened xylem tissue is essentially what we call wood.

New xylem is added each year, forming the annual rings visible in tree trunks.

Phloem is a type of vascular tissue that carries sugars (food) produced during photosynthesis from the leaves to all other parts of the plant, including the roots, stems, fruits and growing regions.

Unlike xylem, phloem is made of living cells. The end walls of phloem cells have small holes (sieve plates) that allow the sugar solution to pass from one cell to the next.

Phloem can carry food both upward and downward depending on where it is needed.

Osmosis is the movement of water molecules from a region of higher water concentration to a region of lower water concentration across a partially permeable membrane.

In plant roots, the concentration of water in the soil is higher than inside the root cells. Because of this difference in concentration, water moves from the soil into the root cells by osmosis.

A partially permeable membrane is a thin layer that allows small molecules like water to pass through but prevents larger molecules like dissolved minerals and sugars from passing freely.

Root hairs are tiny, hair-like extensions of the outer cells of the root. They are very thin-walled and project outward into the spaces between soil particles.

Root hairs are important because they greatly increase the surface area of the root, allowing much more water and dissolved minerals to be absorbed from the soil in a shorter time.

Transpiration is the process by which a plant loses water vapour from its leaves into the surrounding air. This water loss occurs mainly through tiny pores called stomata, which are found mostly on the underside of leaves.

As water evaporates from the leaf cells, it creates a pulling force that draws more water upward through the xylem from the roots. This is called the transpiration stream.

Stomata (singular: stoma) are tiny pores found mainly on the lower surface of leaves. They allow gases (carbon dioxide and oxygen) and water vapour to pass in and out of the leaf.

Each stoma is surrounded by two sausage-shaped guard cells. When the plant has plenty of water, the guard cells become turgid (swollen with water) and the stoma opens, allowing transpiration and gas exchange. When the plant is short of water, the guard cells lose water, become flaccid, and the stoma closes to prevent further water loss.

Temperature: On warmer days, water evaporates more quickly, so the rate of transpiration increases.

Wind: On windy days, water vapour is blown away from the leaf surface, maintaining a large concentration difference that drives faster transpiration.

Humidity: On dry days, the air can hold more water vapour, so transpiration is faster. On humid days, the air is already full of water vapour and transpiration slows down.

Time of day: Stomata are open during the day when photosynthesis occurs. Transpiration is therefore fastest during the day and stops at night when the stomata close.

Photosynthesis is the process by which plants use light energy to convert carbon dioxide and water into glucose and oxygen. It is the process by which plants make their own food.

Word Equation: Photosynthesis takes place in chloroplasts inside leaf cells. Chloroplasts contain a green pigment called chlorophyll that absorbs light energy and drives the reaction.

Glucose produced during photosynthesis is used in five main ways:

1. Respiration: Glucose is broken down to release energy for all the plant's life processes.
2. Cellulose: Glucose is used to make cellulose, which forms strong cell walls.
3. Starch and oils: Excess glucose is converted into starch or oils for storage in roots, stems, seeds and fruits.
4. Protein: Glucose combines with nitrogen obtained from nitrates in the soil to make amino acids and proteins for growth and repair.
5. Chlorophyll: Glucose combines with magnesium absorbed from the soil to make more chlorophyll for photosynthesis.

Respiration is the process by which living cells release energy from glucose. It is not the same as breathing. Respiration happens inside the mitochondria of cells.

Plants respire in all their cells, all the time: day and night: unlike photosynthesis which only occurs during daylight.

Aerobic respiration is the release of energy from glucose using oxygen. The word aerobic means with air (oxygen).

Word Equation: Aerobic respiration releases a large amount of energy and is the most efficient form of respiration.

Anaerobic respiration is the release of energy from glucose without oxygen. It occurs when roots are in waterlogged or flooded soil where no oxygen is available.

Word Equation: Anaerobic respiration releases much less energy than aerobic respiration.