Stern's Introductory Plant Biology - Ch. 5 - Roots and Soils

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Functions of roots

The functions of roots include anchoring the plant in soil, absorbing water and dissolved minerals, storing food and water, conducting materials upward, and forming symbiotic relationships.

Root development

Root development begins when the embryo's radicle emerges from the seed during germination and grows downward to form the first root.

Taproot

A taproot is a single thick, tapered main root with smaller lateral branches, typical of dicots (e.g., carrots, dandelions).

Adventitious roots

Adventitious roots develop from stems or leaves rather than the primary root and form the fibrous root system in monocots (e.g., grasses).

Four regions of root structure

The four regions of a root (from tip outward) are the root cap, region of cell division, region of cell elongation, and region of maturation.

Root cap

The root cap is a thimble-shaped mass of parenchyma cells covering the root tip that protects the delicate meristem from soil abrasion.

Region of cell division

The region of cell division is the apical meristem area at the root tip where rapid mitosis produces new cells.

Protoderm

The protoderm is the outermost primary meristem that gives rise to the epidermis of the root and stem.

Ground meristem

The ground meristem is the primary meristem that develops into the cortex and pith (ground tissues) for storage and support.

Procambium

The procambium is the primary meristem that differentiates into the primary xylem and primary phloem of the vascular cylinder.

Region of cell elongation

The region of cell elongation is where newly produced cells rapidly increase in length by vacuole expansion and water uptake.

Region of maturation

The region of maturation (or differentiation) is where cells specialize into mature types such as root hairs, cortex, endodermis, and vascular tissues.

Root hairs

Root hairs are tubular extensions of epidermal cells in the maturation zone that dramatically increase the surface area for water and mineral absorption.

Cortex

The cortex is the ground tissue layer between epidermis and endodermis, composed mainly of parenchyma cells for storage of starch and transport of water and ions.

Endodermis

The endodermis is the innermost layer of the cortex with cells tightly joined by a waxy Casparian strip that regulates the passage of water and minerals.

Casparian strip

The Casparian strip is a band of suberin and lignin in the radial and transverse walls of endodermal cells that blocks apoplastic flow.

Pericycle

The pericycle is a layer of parenchyma cells just inside the endodermis that can initiate lateral root formation.

Vascular cylinder (stele)

The vascular cylinder (stele) is the central core of the root containing primary xylem and phloem arranged in a star-shaped pattern.

Mycorrhizae

Mycorrhizae are mutualistic symbiotic associations between fungi and plant roots that enhance the absorption of water and minerals.

Root nodules

Root nodules are swellings on the roots of legumes containing nitrogen-fixing bacteria (Rhizobium) that convert atmospheric N₂ into usable forms.

Specialized roots

Specialized roots include storage roots (e.g., sweet potato), prop roots, pneumatophores, and buttress roots.

Soil texture

Soil texture refers to the relative proportions of sand, silt, and clay particles that determine water-holding capacity.

Soil structure

Soil structure is the arrangement of soil particles into aggregates or clumps that influence pore space and water movement.

Soil horizons

Soil horizons are distinct horizontal layers in a soil profile formed by weathering, organic matter accumulation, and leaching.