Chapter 3.2: Roots

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Root Functions

Anchor plants into soil, absorption of water and minerals, store food and water

Root Development

Upon germination, the embryo’s radicle grows out to a primary root and develops into the first root

Tap Roots

In dicots, made up of thinner branch roots

Fibrous Roots

A large number of fine roots of similar diameter, in mostly monocots, but some dicots

Adventitious Roots

Develop from a stem or leaf

Root Cap

Thimble-shaped mass of parenchyma cells covering each root tip, protects tissue from damage as the root grows and secretes mucilage that acts as a lubricant - perception of gravity

Region of Cell Division

Composed of apical meristem and subdivides into three meristematic areas

Protoderm

A component of the region of cell division that gives rise to the epidermis - dermal tissue

Ground Meristem

A component of the region of cell division that gives rise to the cortex and pith - ground tissue

Procambium

A component of the region of cell division that gives rise to primary xylem and primary phloem - vascular tissue

Region of Elongation

Where cells become several times their original length - vacuoles merge

Region of Maturation

Where cells differentiate into various distinctive cell types and root hairs form

Cortex

Made up of parenchyma cells between epidermis and vascular cylinder, mostly stores food

Endodermis

Inner boundary of cortex, consisting of a single-layered cylinder of compact cells - cell walls impregnated with suberin and lignin on radial and transverse walls

Casparian Strips

Waxy region within the endodermis that force water and dissolved substances through the endodermis

Passage Cells

Alternative to Casparian strips

Vascular Cylinder

Core of tissues inside endodermis - most of the cells are primary xylem and primary phloem

Pericycle

Outer boundary of vascular cylinder

Pith

Only in monocots

Exodermis

Mostly in monocots

Epidermis

In monocots and dicots, covers the root and aids in absorption

Secondary Growth

Occurs through pericycle and lateral meristem

Intermediate Growth

New tissues are added indefinitely, season after season

Food Storage Roots

Starch and other carbohydrates, sweet potatoes

Water Storage Roots

Pumpkin family, especially in arid regions

Pneumatophores

In plants with roots growing in water, spongy roots that extend above the water’s surface and enhance gas exchange between atmosphere and subsurface roots

Aerial Roots

Orchids, corn, English ivy, Virginia creeper

Contractile Roots

Pull the plant deeper into the soil, dandelions

Buttress Roots

Provide stability in shallow soil, tropical trees

Mycorrhizae

Fungi that form a mutualistic association with plant roots, plants develop fewer root hairs and are particularly susceptible to acid rain

Mutualistic Association

Both fungus and root benefit and are dependent upon association for normal development

Root Nodules

A few species of plants contain large numbers of nitrogen-fixing bacteria - legumes

Root Nodules Bacteria

Bacteria produce enzymes that convert nitrogen which are readily absorbed by the roots - rhizobium and cyanobacteria