Plant Root Structure and Function

• Plant tissues form organs, such as leaves, stems, or roots and each has specific functions.
• Vascular plants possess a shoot system and a root system.

The Shoot System

• Composed of vegetative parts (leaves and stems) and reproductive parts (flowers and fruits).
• It typically grows above ground to absorb light for photosynthesis.

The Root System

• Anchors the plant below ground.
• Absorbs water and minerals.
• Acts as a storage site for food.

Roots

• Roots are typically underground plant organs that anchor, transport water, minerals, and sugars.
• Roots store carbohydrates when needed, and their depth can vary.

External Root Structure

• Two main types of root systems exist: fibrous and taproot systems.
• Fibrous root systems have branching roots forming a dense mass without a visible primary root, common in monocots.
• Taproot systems have a main root branching into lateral roots, common in eudicots.
• Some plants combine taproots and fibrous roots.
• Plants in dry areas have deep roots, while those in wet areas have shallower roots.
• Primary roots originate from the seedling root, secondary (lateral) roots from the primary roots, and adventitious roots from stems or leaves.
• Adventitious roots can grow from plant cuttings placed in water.

Internal Root Structure

• Root growth starts with seed germination with the radicle growing downward to form the root system.
• Root tips have a protective root cap.
  • The root cap is continuously replaced to prevent damage as the root pushes through the soil.
• The root tip consists of three zones: cell division, elongation, and cell maturation.
  • The zone of cell division is a continuation of the root cap made of actively dividing cells
  • The zone of elongation is where newly formed cells lengthen
  • The zone of cell maturation is where root cells differentiate into special cell types beginning at the first root hair
• Root hairs increase the surface area for water and mineral absorption.
• The epidermis, an outer layer of cells, surrounds ground tissue and vascular tissue.

Cross Section

• Ground tissue in roots forms two regions: the cortex and the pith.
• Roots compared to stems, have more cortex and little pith.
  • Eudicot roots lack a central pith.
  • Monocots possess a small pith.
• Both cortex and pith store photosynthetic products.
• The stele contains vascular tissue, including xylem and phloem
  • The endodermis borders the stele and is exclusive to roots.
    • It functions as a checkpoint for materials entering the root’s vascular system.
    • The Casparian strip, a waxy region, forces water and solutes to cross endodermal cell membranes.
  • The pericycle is the outermost cell layer of vascular tissue, capable of forming lateral roots.
• Monocots feature vascular tissue arranged in a ring around the pith and the cortex surrounds the stele.
  • The endodermis is the innermost layer of the cortex, and the exodermis is the outermost.
• Eudicot roots feature vascular tissue filling the center with no pith.
  • The xylem and phloem of the stele are arranged alternately in an X shape.
  • Most of the root is cortex tissue, with the endodermis bordering the stele.

Secondary Root Growth

• Many roots undergo both secondary and primary growth via meristemic tissue production.
• The cork cambium increases the girth of the root, adding vascular tissue.
• The vascular cambium forms secondary xylem inside and secondary phloem outside.

Root Modifications

• Roots can be modified for specific purposes like bulbous roots that store starch.
• Taproots in carrots, turnips, and beets are modified for food storage.
• Aerial roots provide additional support to anchor plants.
  • Epiphytic roots allow plants to grow on other plants.
• Contractile roots shorten to pull the plant deeper into the sand.
• Stilt roots, found in mangroves, are supportive aerial roots.
  • Stilt roots are adventitious and grow from lateral branches.
• Pneumatophores are specialized roots that allow oxygen and carbon dioxide exchange
• Root nodules on nitrogen-fixing plants contain bacteria to deoxidize atmospheric nitrogen into ammonia
  • N2 is converted to NH3
• Mycorrhiza enhances mineral and water absorption through fungus penetration.

Stems

• Consist of the stem and leaves and their functions include support, conduction, photosynthesis, and storage.

Stem Morphology

• Stems have nodes and internodes that are present above and below ground,
  • Nodes are points of attachment for leaves.
  • Internodes are the stem region between two nodes.
• Axillary buds are located where the leaf base meets the stem, giving rise to axillary shoots or branches.
• The shoot apex at the tip contains the shoot apical meristem surrounded by developing leaves called leaf primordia.

Internal Anatomy of the Primary Stem

• The herbaceous comprises the non-woody stem which has not undergone secondary growth.
• All three tissue types are in the primary stem.
  • The epidermis is the dermal tissue layer that surrounds and protects the stem.
    • It has a waxy cuticle and stomata for gas exchange which is regulated by guard cells.
    • Trichomes aid in reducing transpiration, increasing solar reflectance, and deterring herbivores.
  • Ground tissue forms the cortex within the epidermis and the pith in the center.
• Cross-sections show three arrangements of vascular tissue in the stem.

Secondary Stem

• Primary growth increases stem length through cell division in the shoot apical meristem.
• Secondary growth increases thickness, and it is caused by cell division in secondary meristems.
  • Secondary growth is common in woody plants but rare in monocots.
• Lateral meristems (vascular and cork cambium) drives secondary growth.
  • The vascular cambium produces secondary vascular tissue, secondary xylem (internally) and secondary phloem (externally).
  • The cork cambium produces phelloderm (internally) and cork (externally).

Periderm and Bark

• The phelloderm, cork cambium, and cork form the periderm, the dermal tissue of the secondary plant body
  • Bark includes all tissues external to the vascular cambium, which protects against damage and reduces water loss.
    • The inner bark is inside the cork cambium.
    • The outer bark includes the cork cambium and everything external to it.

Wood

• Wood consists of the secondary xylem produced by the vascular cambium.
• Heartwood stores compounds and appears darker than surrounding wood.
• Sapwood surrounds the heartwood, appearing lighter and consisting of conducting xylem.
• Dendrochronology studies annual rings, which consist of:
  • Wide tracheary elements form in spring to transport high water volumes.
• During summer, narrow tracheary elements occur due to lower water availability
  • Thick rings indicate wet years, and thin rings indicate dry years.

Stem Modifications

• Rhizomes and stolons are horizontal stems for propagation.
• Rhizomes are belowground stems.
• Stolons are aboveground horizontal shoots and they produce new plants.
• Tubers, corms, and bulbs store starch.
  • Tubers which include potatoes, are thick, belowground stems and found the tips of rhizomes or stolons
  • Corms are short, thick underground storage stems with thin, scaly leaves.
  • Bulbs, such as onions, store nutrients in fleshy leaves.
• Some plants have thorns, prickles, and spines for defense against herbivores.

Leaves

• Leaves are specialized organs of the shoot for photosynthesis.
• Leaves exhibit a determinate growth pattern, ceasing growth at a certain size.
• Leaves originate from the shoot apical meristem through leaf primordia.

Internal Leaf Structures

• All three tissue types are in leaves.
• The epidermis is dermal tissue.
• The mesophyll is ground tissue.
• Vascular bundles form leaf veins comprising vascular tissue.
• Mesophyll cells have chloroplasts and specialize in photosynthesis.
  • Most leaves: palisade and spongy parenchyma cells
    • Palisade parenchyma specialize in capturing incoming sunlight.
    • Spongy parenchyma assist in gaseous exchange.
• Mesophytes adapt to moderate water amounts.
• Hydrophytes grow in water.
  • They have dissected leaf blades and air canals in petioles/stems for gas exchange.
• Xerophytes adapt to scarce water.
  • They have thick cuticles to limit water loss.

Leaf Modifications

• When the leaf blade isn't primarily for photosynthesis, other plant parts take over its role.
• Storage leaves store starch underground.
• Succulent leaves primarily store water, and conduct photosynthesis.
• Spines are modified leaves function in plant defense.
• Tendrils are used for climbing plants to attach to nearby structures for support.
• Carnivorous plant leaves are traps to capture insects.