Plant Anatomy – Roots (Primary Structure & Function)

Vocabulary flashcards covering key terms, tissues, and concepts from the lecture on primary root structure, function, and development.

Root

The below-ground organ of a plant that anchors, absorbs water and minerals, stores carbohydrates, and serves as a conduit to the shoot.

Primary Root

A young root that has not yet undergone secondary thickening; includes fine, absorptive roots at the tips of the system.

Secondary Growth

Thickening of roots or stems via vascular and cork cambia, producing wood and bark.

Fine Roots

Thin, short-lived roots (often <2 mm diameter) responsible for most water and nutrient uptake.

Root Hair

A single-cell tubular extension of an epidermal cell that greatly increases root surface area for absorption.

Rooting Depth

The maximum depth a plant’s roots penetrate; a key ecological trait influencing water access and community composition.

Hydraulic Lift

Upward movement of water from deep, moist soil layers to drier surface layers through roots, especially at night.

Hydraulic Redistribution

Bidirectional movement of water within the root system, allowing rapid downward flow after rainfall or upward lift during drought.

Mycorrhiza

Symbiotic association between plant roots and fungi in which fungal hyphae enhance mineral (especially phosphorus) uptake.

Root Nodule

Specialized root structure housing nitrogen-fixing bacteria (e.g., Rhizobium, Frankia) that supply the plant with usable nitrogen.

Frankia

A genus of actinobacteria that forms nitrogen-fixing nodules on non-legume hosts such as Coriaria and alder.

Rhizobium

Nitrogen-fixing bacteria that form nodules specifically on legumes.

Root Cap

Protective group of cells at the root tip that secretes mucilage, senses gravity, and sloughs border cells as the root grows.

Meristematic Zone

Region just behind the root cap where actively dividing cells (initials and derivatives) produce new root tissues.

Zone of Elongation

Root region where cells expand lengthwise, pushing the tip through soil.

Zone of Maturation

Area where root cells complete differentiation and root hairs are abundant.

Protoderm

Primary meristem that gives rise to the epidermis.

Ground Meristem

Primary meristem that differentiates into cortex (and sometimes pith) tissues.

Procambium

Primary meristem that forms the vascular tissues (xylem and phloem) and pericycle.

Stele (Vascular Cylinder)

Central root region containing xylem, phloem, pericycle, and sometimes pith; also called the vascular cylinder.

Phloem

Vascular tissue responsible for translocating sugars and other organic molecules; matures first in young roots.

Xylem

Water-conducting vascular tissue composed of tracheary elements; matures after phloem in roots.

Protoxylem

First-formed xylem with extensible secondary wall patterns (spiral, annular) that allow continued elongation.

Metaxylem

Later-formed xylem with rigid, pitted secondary walls; larger diameter than protoxylem.

Exarch Xylem Development

Maturation pattern in roots where protoxylem is exterior and metaxylem interior (centripetal).

Endarch Xylem Development

Maturation pattern in stems where protoxylem is interior and metaxylem exterior (centrifugal).

Cortex

Ground tissue between epidermis and endodermis, often parenchymatous and starch-rich.

Rhizosphere

Soil zone directly influenced by root secretions and associated microbiota.

Endodermis

Innermost cortical layer whose cells possess a Casparian strip, regulating apoplastic flow into the stele.

Casparian Strip

Suberin/lignin impregnated band in radial walls of endodermal cells that blocks uncontrolled apoplastic transport.

Exodermis

Secondary, suberized outer cortical layer in some roots that reduces water loss and pathogen entry.

Pericycle

Layer of parenchyma just inside the endodermis that retains meristematic capability, giving rise to lateral roots and vascular cambia.

Protostele

Stele type with a solid core of xylem (often star-shaped) surrounded by phloem; common in eudicot roots.

Siphonostele

Stele type with xylem and phloem forming a ring around a central pith; typical of many monocot roots.

Polyarch

Having many protoxylem poles (≥6); characteristic of monocot roots with siphonosteles.

Lateral Root Primordium

Developing meristematic bulge originating in the pericycle that will emerge as a new root branch.

Vascular Cambium

Secondary meristem derived partly from pericycle that produces secondary xylem (wood) inward and secondary phloem outward.

Cork Cambium (Phellogen)

Secondary meristem that forms protective periderm (cork) replacing the epidermis in thickening roots or stems.

Border Cells

Root-cap cells that detach and secrete mucilage, aiding soil penetration and influencing rhizosphere interactions.

Mucilage

Hydrated polysaccharide gel secreted by root-cap and border cells, lubricating root passage through soil.