Plant Science - Organography of Plants: Roots

Organography of Plants: Roots

Introduction

  • In vascular plants, the root is typically below the soil surface but can be aerial or aerating.
  • Roots lack leaves and nodes.
  • The first root from a plant is the radicle.

Origin and Characteristics of Roots

  • Growing tip is protected by the calyptra.
  • Roots do not bear leaves and axillary buds, though auxiliary buds may develop.
  • Roots are not divided into nodes and internodes.
  • Roots theoretically possess unlimited growth in length.
  • Vascular tissue is radially arranged.
  • Vascular tissue in stems is collaterally or bicollaterally arranged in vascular bundles.

Anatomy of the Root

  • Roots consist of three concentric rings of tissues:
    • Vascular tissue
    • Ground tissue
    • Epidermis tissue

Root Growth

  • Early root growth begins at the apical meristem near the root tip.
  • Meristem cells divide, producing new meristem cells, root cap cells, and undifferentiated root cells.
  • Cells elongate, pushing the root tip forward, and differentiate into specialized root tissues.
  • Roots grow towards environments with adequate air, nutrients, and water, but not in dry soil.
  • During germination, roots grow downward due to gravitropism; shoots grow upward.
  • Roots can cling to walls or structures (e.g., ivy).
  • Primary growth (apical meristem) involves elongation.
  • Secondary growth refers to an increase in diameter.
  • Active cell division occurs in the first 5 mm of the root tip, protected by the calyptra.
  • Cell elongation region: cells grow to their full size.
  • Cell differentiation region: root hairs form for water and nutrient absorption.

Root Systems

  • Types are identified based on origin and/or external morphology.
  • Two basic types:
    • Taproot system
    • Adventitious root system

Tap Root System

  • Most common type; consists of a taproot (primary root) that grows vertically downward (positive geotropism).
  • Lateral roots (secondary roots) grow horizontally or diagonally.
  • Secondary roots produce tertiary roots.
  • Many orders of roots descend in size from the taproot.
  • Most dicotyledonous plants produce taproots (e.g., Phaseolus vulgaris).
  • Taproot systems usually penetrate deep into the soil.
  • In some cases, taproot systems are modified into fibrous systems, where secondary roots equal or exceed the primary root in size.
  • Fibrous root systems have no well-defined single taproot and are generally shallower than taproot systems.

Adventitious Root System

  • Commonly found in representatives of the Liliidae (monocotyledonous plants).
  • Primary root usually has a limited lifespan and is replaced by adventitious roots.
  • Adventitious roots originate from any plant organ other than a root itself (e.g., stem cuttings of Pelargonium incanum).
  • Adventitious roots usually form a shallow root system.
  • Most common garden weeds are dicotyledonous plants with taproot systems, while grasses are monocotyledonous plants with adventitious root systems.

Root Types

  1. Taproot (main root/primary root)
  2. Adventitious root
  3. Lateral root (secondary root): Originates laterally from any other root.
  4. Hair root: Thin, multicellular, hair-like lateral root.
  5. Root hair: Unicellular outgrowth of an epidermis cell near the growing tip; enlarges the surface area for absorption.
  6. Fibrous roots: Commonly found in grasses; usually adventitious roots of the same length and thickness; feels hard and fibrous.

Functions of Roots

  1. Absorption of water and inorganic nutrients.
  2. Anchoring of the plant body to the ground.
  3. Storage of food and nutrients.
  • Absorption mainly occurs in the region of the root hair shortly beyond the growing tip.
  • Roots transport absorbed nutrients and water to the stem.

Metamorphic (Modified) Roots

  • Modified to fulfill an exceptional function beyond the normal functions of roots.
  • Have a changed or modified morphology and do not look like “normal” roots.
  • Plants conserve energy resources for future growth in locations with lengthy dry seasons or competition for resources.

Roots as Storage Organs

  • Underground storage organs are mostly found in monocot plant families Iridaceae, Liliaceae, and Amaryllidaceae.
  • These energy pools are large and fleshy to store food for new growth.
  • Most common types:
    • Bulbs: store the entirety of the plant's life cycle underground to ensure survival.
    • Stem Tubers: differ from bulbs or corms by not having a basal plate or protective tunic.
    • Corms: modified stem base with stored tissue on top of its enlarged base; storage rings are not visible when split.
    • Rhizomes: develop horizontally beneath the soil's surface.
    • Some plants simply have larger, fleshy roots.

Aerial Roots

  • Fulfill a specialized function above the soil surface; develop from the aerial part of the plant and are thus adventitious roots.
    • Prop roots: Adventitious roots develop from the stem above the soil surface, grow downward, reach the soil, and support the plant (e.g., Zea mays, Pandanus kirkii).
    • Buttress roots: Aerial extensions of lateral surface roots; stabilize the tree, especially in shallow saturated soils; flattened adventitious roots growing horizontally from the stem (e.g., Ficus sycomorus, Ficus elastica).
    • Twining and/or climbing roots: Adventitious and/or lateral roots that twist around a support to keep the plant upright; a velamen is often present.
    • Crampons: Occur in epiphytes and climbers; groups of short adventitious roots growing negatively phototropically to attach the plant to a wall or other substrate; root tips are usually flattened for better attachment.
    • Strangling roots: Occur in some hemi-epiphytes; adventitious and lateral roots that twine around the support plant and fuse where they touch, ultimately enclosing the support plant completely (e.g., Ficus thonningii, Ficus burkei).

Aerial Roots with Velamen

  • Generally found in epiphytes; the velamen is a modified epidermis (several layers thick).
  • Mature cells are dead, and their cell walls contain a reticulate thickening.
  • The velamen has a sponge-like texture and can absorb moisture and nutrients from the atmosphere.
  • Humus can also be trapped in the mass of roots.
  • Active growing parts of the roots are often green and can photosynthesize (e.g., Ansellia gigantia, Orchidaceae).
  • Epiphytes grow on other plants, and their roots are not connected to the soil.
  • Epiphyte roots developed a unique and ingenious mechanism for rainwater capture because the only opportunity these roots (aerial roots) have to obtain most of their moisture is when it rains, which, in the tropics occurs as intense, short downpours.
  • Velamen consists of dead cells, usually several layers thick, covering the outer surfaces of aerial roots

Root Thorns

  • Occur in aerial roots.
  • The calyptra and/or the growing tip is hardened into a thorn-like structure.

Pneumatophores (Breathing Roots)

  • Occur in mangroves.
  • Not easily recognizable as roots but suited to their specific habitat.
  • Two types:
    • Rod-like pneumatophores: Exhibit negatively gravitropous growth (e.g., Avicennia marina).
    • Knee-roots: Some roots grow out above the low tide water level and curl downwards again into the mud; lateral roots with positive gravitropous growth originate to support the knee-root (e.g., Bruquiera gymnorrhiza).
  • Pneumatophores have porous areas (pneumatodes) connected to other porous tissues in the root.
  • Exchange of gases occurs through the pneumatodes, diffusing to deeper-lying roots.
  • The phenomenon of biotechnosis frequently occurs in Bruguiera gymnorrhiza, the hypocotyl elongates (can become up to 1 m in length), breaks off and pegs in the mud below.
  • Examples: Avicennia marina and Bruguiera gymnorrhiza.

Contractile Roots

  • Possess the ability to reduce (shrink) in length.
  • Keep the organ (bulb or rhizome) to which they are attached at a constant depth in the soil.
  • Examples: Crinum bulbispermum, Hypoxis rooperi, and Iris xiphium.

Roots of Parasitic Plants

  • The part of the parasitic root that penetrates into the host is known as a haustorium (sucking organ).
  • In stem parasites, the seed lands on the stem of the host plant, germinates, and the parasitic root penetrates the host tissue.
  • A connection between the vascular tissue of the parasite and the host is established.
  • Root parasites attach to the host root.
    • Holoparasite: Completely parasitic on other plants and has virtually no chlorophyll (e.g. Cuscuta campestris).
    • Hemiparasite: Parasitic under natural conditions and is also photosynthetic to some degree (e.g. Tapinanthus sp.).
  • In root parasites, the seed is often dormant in the soil; a root secretion of the host stimulates the seed to germinate.
  • As soon as the parasitic roots reach the host, the haustoriums penetrate the roots of the host.
  • Parasitic plants often also have "normal" roots.
  • Examples: Alectra sp. and Striga asiatica.