Plant structure

Like multicellular animals, plants have organs composed of different tissues, which in turn are composed of cells. All angiosperms consists of a root system and a shoot system

Root systems:

1. Taproot System: A root system consisting of one prominent main root with smaller lateral roots branching from it
2. Fibrous root system: A root system consisting of several adventitious roots of approximately equal size that arise from the base of the stem. A fibrous root system is universal in monocotyledonous plants and ferns

Importance of root system:

1. Absorption of water and minerals from the soil
2. anchorage in soil
3. storage of energy resources
4. conduction of water and minerals to and from the shoot.

Taproot System

The taproot system has a main central root upon which small, lateral roots called root hairs are attached. The radicle of many plant species dies soon after maturation, hence, the taproot gets converted into the fibrous root system in the later stages.

A taproot system consists of one thick, cylindrical, main root known as the primary root. Branches of roots arise from the primary roots and these branches are known as secondary and tertiary roots. At the tip of the tertiary root, a thin hair-like structure is present which is known as the 'rootlet'. The rootlets are critical for water absorption from the soil.

The taproot system is generally deep-rooted and difficult to uproot. They are observed in dicots.

Taproots are found in:

• Carrot
• Dandelion
• Turnip
• Beetroot
• Coriander
• China rose
• Radish
• Parsley
• Mustard

Fibrous Root System

The fibrous root system consists of bushy roots that develop from the stem in the form of thin and moderately branching roots without any primary root.

The fibrous root system begins as a taproot from the radicle, but as the plant grows, the radicle degenerates and no primary root is seen.

Once the plant is fully developed, the fibrous root system appears as a mat underneath the plant. The roots in the fibrous root system move more horizontally than vertically, but they cannot penetrate deeper into the soil.

Fibrous roots are considered surface feeders and cannot act as organs for food storage. They are found in monocotyledons such as:

• Rice
• Corn
• Wheat
• Marigold
• Banana
• Coconut

Adventitious Root System

Adventitious roots are roots that arise from parts of the plant other than the radicle. These roots can emerge from the injured root, nodes of the stem, internodes, branches, or any other tissue.

Instead of having one primary root from which the branches arise, an adventitious root system consists of numerous morphologically similar roots that grow from the same node.

Adventitious roots are generally seen growing from the aerial parts of plants. They can develop from leaf and stem cuttings when placed in soil. These roots are formed from root primordial cells and are found in monocotyledonous plants.

Adventitious roots are modified for various purposes such as mechanical support, climbing, clinging, and performing other important functions like food storage and providing structural support for plants.

Root Structure

Root Cap (Calyptra)

The root cap is a protective structure that resembles a cap and is located at the growing tip of the root. It secretes mucilage, which serves as a lubricant, aiding the root's movement through the soil. Mucilage also facilitates the uptake of water and nutrients. As the root extends further into the soil, the root cap continuously degrades and renews itself, protecting the meristem beneath it.

Growing Point or Meristematic Zone

This zone is situated a few millimeters behind the root cap. The root meristem is responsible for adding cells to both the root cap and the basal region of the root.

The meristem at the apex consists of three layers:

• Dermatogen: The outermost layer, whose cells mature into the epiblema and root cap.
• Periblem: Located inner to the dermatogen, its cells mature into the cortex.
• Plerome: The central region, where cells mature into the stele.

Zone of Elongation

This zone is located a few millimeters behind the meristematic zone and is characterized by rapid and extensive cell elongation, contributing to the overall length of the root. The outer cells in this zone absorb water and minerals from the soil.

Root Hair Zone or Zone of Differentiation

This zone is where cells differentiate to form the epiblema, cortex, endodermis, pericycle, xylem, and phloem. Many epiblema cells elongate to create unicellular root hairs. As the root grows, new root hairs develop while older ones wither, thereby increasing the absorptive surface area of the root.

Zone of Maturation

This zone constitutes the major portion of the root, where cells reach maturity. It is also the site for the production of lateral roots.

Shoot System

The shoot system consists of several organs, including the stem, branches, petioles, buds, leaves, flowers, seeds, and fruits.

Functions of the Shoot System

• Support and Transportation: Provides structural support and facilitates the movement of nutrients and water throughout the plant.
• Gaseous Exchange: Allows for the exchange of gases necessary for respiration and photosynthesis.
• Sexual Reproduction: Involves the formation of flowers and seeds for reproduction.
• Photosynthesis: Conducts photosynthesis to convert light energy into chemical energy.

Stem

Stems are a vital component of the shoot system in plants. They are typically found above the ground, although some plants, like potatoes, have stems that grow underground.

A stem can be either unbranched, as seen in a palm tree, or highly branched, like in a magnolia tree.

Stems are divided into two main parts: nodes and internodes. Nodes are the regions where branching occurs, while internodes are the sections between nodes where no branching takes place. The nodes serve as points of attachment for leaves, aerial roots, and flowers. The stalk that connects the stem to the base of a leaf is known as the petiole.

Functions of a Stem

• Support: Provides structural support to the plant.
• Reproduction: Bears flowers and fruits in positions that facilitate pollination, fertilization, and seed dispersion.
• Asexual Reproduction: Some stems are modified for vegetative propagation, a form of asexual reproduction.
• Attachment: Holds plant buds, flowers, leaves, and fruits.
• Photosynthesis: Few green stems contain chloroplasts and can carry out photosynthesis.
• Food Storage: Acts as a storage area for prepared food, often in the form of starch.
• Connection: Connects the roots to the leaves.
• Structural Foundation: Provides a sturdy structure for the plant, which can develop into a tree.
• Transportation: Transports water and minerals from the roots to other parts of the plant.

Underground Stem

Underground stems remain at ground level and produce aerial shoots that rise above the soil. They serve the purposes of food storage and perennation, and they are also capable of vegetative propagation.

Types of Underground Stem

• Tuber: A horizontal underground stem that enlarges at its growing tips due to the accumulation of stored food. An example is the potato.
• Bulb: A short underground stem with a fleshy base and leafy scales. The stem is reduced to form a disc-like structure, storing food in modified leaves. Examples include lily and onion.
• Corm: A short, upright, swollen underground stem that acts as a food storage organ, helping the plant survive adverse conditions. Examples include colocasia, taro, gladiolus, and crocus.
• Rhizome: A horizontal, thickened underground stem that has distinct nodes and internodes, along with scaly leaves at the nodes. Examples include ferns, ginger, nettles, asparagus, lily of the valley, and turmeric.

Subaerial Stem

Subaerial stems run parallel to the ground and give off shoots at certain intervals or nodes. They are partially aerial and partially underground.

Types of Subaerial Stems

• Runner: This type grows parallel to the ground and features a creeping stem with long internodes. Runners extend horizontally on the soil surface and bear scale leaves and adventitious roots. They typically develop from the axils of lower leaves on the aerial stem. Examples of runners include spider grass, peppermint, bermudagrass, strawberry, doob grass, oxalis, and lawn grass.
• Stolon: Similar to a runner, a stolon arises from the lower part of the main axis and is a weak lateral stem. After growing aerially for some time, it bends downwards to touch the ground, where its terminal bud produces a new shoot and adventitious roots. Examples of plants that spread using stolons include jasmine, colocasia, and mint.
• Sucker: This type rises from the basal underground part of the main stem. It grows horizontally for a distance below the soil before growing upwards obliquely. The stem develops a leafy shoot and adventitious roots before separating from the mother plant. Examples of plants that use suckers include chrysanthemum, mentha, apple, elm, and banana trees.
• Offset: Shorter and thicker than a runner, offsets are often found in aquatic plants. They have one internode and develop from a leaf axil, growing as a short horizontal branch that produces a rosette of leaves above and adventitious roots below. This type is observed in plants such as pistia, Eichhornia, and Centella.