ROOTS

Primary Functions of Roots

  • The radicle, the first structure to emerge from a germinating seed, is the primary root. It grows downwards into the soil, providing initial anchorage for the seedling.

  • The root system has several vital functions:

    • Anchoring the plant, providing stability.

    • Absorbing water and nutrients from the soil, which are essential for plant growth.

External Structure of Roots

  • Fibrous Root System:

    • Characterized by a mat of thin, extensively branched roots that typically spread out below the soil surface.

    • Common in monocots (e.g., grasses, often developed from adventitious roots).

  • Taproot System:

    • Develops from the embryonic root, the radicle.

    • Consists of one large, prominent vertical root (the taproot) that grows deep into the soil.

    • Numerous smaller lateral roots branch off the taproot.

    • Common in dicots (e.g., carrots, most trees, beans).

  • Adventitious Roots:

    • Roots that originate from stem tissue or leaves, rather than from another root.

    • Can be present in both monocots and dicots (e.g., prop roots of corn).

Structure of Individual Roots

  • General Regions of Growth (from tip upwards):

    • Root Cap: Protective covering at the very tip.

    • Region of Cell Division: Contains the apical meristem, where new cells are constantly produced by mitosis.

    • Region of Elongation: Cells rapidly increase in size, pushing the root tip deeper into the soil.

    • Region of Maturation (Root Hair Zone): Cells differentiate into specialized tissues; root hairs are formed here.

  • Vascular Tissue: Present throughout the root, responsible for transport.

  • Lateral Roots: Originate from deeper within the root, typically from the pericycle.

  • Root Hairs: Fine, hair-like extensions of epidermal cells, found in the region of maturation.

Detailed Internal Structure of Roots

  • Root Cap:

    • Composed of parenchyma cells.

    • Secretes mucigel, a slimy polysaccharide that lubricates the root's passage through the soil, reducing friction.

    • Plays a role in the perception of gravity (gravitropism).

    • Slough Cells: Outer cells of the root cap are continuously shed as the root grows.

    • Quiescent Center: A central region of mitotically inactive cells within the root cap, surrounded by active meristematic tissue.

      • This region can become active and form a new apical meristem if the surrounding tissue is damaged.

      • It is resistant to harmful agents like radiation and toxic chemicals, serving as a reserve of healthy cells.

  • Zone of Cell Division:

    • Located just behind the root cap.

    • Contains the apical meristem at its center.

    • Cells in this region divide frequently, approximately every 12 to 36 hours, producing new cells for the root cap and the growing root body.

  • Zone of Elongation:

    • Located above the zone of cell division.

    • Cells expand significantly in length, which accounts for the most significant portion of root growth.

    • Some meristematic activity may continue, but expansion is dominant.

    • Outermost cells are protoderm, which differentiate into the epidermis.

    • The center contains provascular tissues, which will develop into primary xylem and primary phloem.

    • Tissues in this zone are quite permeable, allowing minerals to penetrate.

    • This zone is relatively short, so little actual absorption occurs here.

    • Lack of root hairs or lateral roots in this zone prevents shearing forces during growth through soil particles.

  • Zone of Maturation / Root Hair Zone:

    • Located above the zone of elongation.

    • The primary region for cell differentiation.

    • Root hairs are abundant here:

      • They are not separate cells but are tubular extensions of specialized epidermal cells.

      • They hold tightly to soil particles.

      • They dramatically increase the surface area for water and mineral absorption (up to 250,000 per in^2).

      • Root hairs form only above the elongation zone.

    • This zone is crucial for the efficient transfer of minerals from the epidermis to the vascular tissue.

Diffusion Paths in Roots

Water and minerals can move through the root epidermis and outer cortex via a combination of pathways:

  1. Apoplastic Pathway: Water and minerals diffuse exclusively through the non-living components of the root—cell walls and intercellular spaces.

  2. Symplastic Pathway: Material passes through a plasma membrane and then moves from cell to cell via protoplasts (connected by plasmodesmata).

  3. Apoplast to Symplast: Materials initially move apoplastically then cross a plasma membrane to enter the symplast.

  4. Symplast to Apoplast: Materials might move symplastcially then exit to the apoplast.

Monocot and Dicot Root Cross Sections

  • Three Primary Tissues: All roots have an Epidermis (dermal tissue), Cortex (ground tissue), and Stele (vascular tissue).

  • Cross Section of a Dicot Root:

    • Epidermis: Outermost protective layer.

    • Cortex: Lies beneath the epidermis.

    • Stele (Vascular Cylinder): Central portion containing the vascular tissues.

      • Endodermis: Innermost layer of the cortex, surrounding the stele.

      • Pericycle: Layer inside the endodermis, giving rise to lateral roots.

      • Xylem: Forms an irregular, solid strand or an X-shape (often described as star-shaped).

      • Phloem: Arranged in separate strands between the arms of the xylem.

    • No Pith in the center, unlike monocots.

    • Examples: Carrots, gumamela, beans, most trees.

  • Cross Section of a Monocot Root:

    • Epidermis, Cortex, Endodermis, Pericycle similar to dicots.

    • Stele (Vascular Cylinder):

      • Separate strands of xylem and phloem alternate around a central region.

      • Pith: A large, parenchyma-filled central core is present, surrounded by the vascular bundles.

    • Examples: Grasses, grains, corn.

Endodermis and Casparian Strip

  • The endodermis is the innermost layer of the cortex cells.

  • Casparian Strip: A waxy, waterproof band made of suberin (a fatty substance) embedded in the cell walls of endodermal cells.

    • It prevents water and solutes from moving through the apoplastic pathway (between the cells) as they approach the vascular cylinder.

    • This forces water and solutes to cross the plasma membrane of endodermal cells and enter the symplastic pathway.

    • This selective process ensures that only materials required by the root pass into the stele, selectively excluding toxic substances and pathogens.

The Pericycle

  • The pericycle is located just inside the endodermis.

  • It plays crucial roles:

    • Lateral Root Growth: Facilitates and regulates the initiation and growth of new lateral roots, particularly numerous in fibrous root systems.

    • Secondary Growth: In dicots and gymnosperms, pericycle cells contribute to the formation of the vascular cambium and cork cambium, stimulating secondary growth (thickening of roots and stems).

Other Types of Roots and Root Modifications

  • Storage Roots: Enlarged roots that store carbohydrates (like starch) or water.

    • Food-storage roots: Sweet potatoes, carrots, beets, turnips.

    • Water-storage roots: Found in arid regions, e.g., plants of the squash family.

  • Prop Roots:

    • Provide extra support for tall plants or those in unstable environments.

    • Examples: Mangroves (Rhizophora ext{ }sp.), corn (Zea ext{ }mays), screw pine (Pandanus ext{ }utilis).

    • The structural design of prop roots, particularly in screw pines, inspired the architectural flying buttresses.

  • Pneumatophores:

    • Specialized aerial roots that grow upwards from submerged roots.

    • Function in gas exchange (oxygen absorption) in waterlogged soils where oxygen is scarce.

    • Example: Black mangrove (Avicennia ext{ }germinans).

  • Buttress Roots:

    • Large, triangular plate-like roots that grow vertically from the base of tall tropical trees.

    • Provide significant structural support and stability, preventing the tree from toppling.

    • Inspired the design of buttress walls in Gothic cathedrals.

  • Aerial Roots:

    • Roots that hang in the air, often covered by a specialized epidermis.

    • Example: Orchids have aerial roots covered by a white spongy tissue called velamen, which helps absorb and retain water from the air.

  • Contractile Roots:

    • Roots that can shorten, pulling the plant stem or bulb deeper into the soil to maintain optimal depth.

    • Example: Contractile roots of a hyacinth.

  • Mycorrhizae (Fungus Roots):

    • A mutualistic symbiotic association between plant roots and fungi.

    • The fungus grows on and sometimes into the roots but usually cannot penetrate the Casparian strip.

    • Fungus benefits: Receives sugars (carbohydrates) from the plant.

    • Plant benefits: Receives enhanced absorption of phosphorus, water, and other minerals due to the fungus's extensive hyphal network.

  • Root Nodules and Nitrogen Fixation:

    • Small swellings on the roots of certain plants, particularly legumes (e.g., beans, peas).

    • These nodules are filled with nitrogen-fixing bacteria, commonly Rhizobium.

    • The bacteria absorb atmospheric nitrogen (N2) and convert it into ammonia (NH3), a form that the plant can use to synthesize amino acids and other essential nitrogenous compounds.

  • Haustorial Roots (Parasitic Roots):

    • Specialized roots of parasitic flowering plants.

    • Haustoria (singular: Haustorium): Projections that develop along the stem where it contacts a host plant.

    • These haustoria penetrate the outer tissues of the host and connect with its vascular tissue (xylem and phloem), allowing the parasite to draw water, minerals, and nutrients from the host.

    • Example: Mistletoe.