Roots
External Structure of Roots
Organization of Root Systems
Most plants have a highly branched root system
Single prominent taproot that is larger than all the rest
Numerous lateral roots/branch roots
Develops from the radicle
Fibrous root system
radicle dies during or immediately after germination
root primordia at base of radicle produce adventitious roots
Adventitious roots increase absorptive and transport capacities of the root system
Taproot vs. Fibrous root systems
Monocots cannot undergo secondary growth, so Fibrous root system is better for them
Eudicots undergo secondary growth in root and stems, allowing the utilization of taproot system
Structure of Individual Roots
Root tip
Region where growth in length occurs
Root Apical Meristem
Protected by the Root Cap
Dictysomes produce mucigel
Lubricates passage of root through soil
Zone of elongation
Behind the root cap and root apical meristem
Cells undergo division and expansion
Root hair zone
Root hairs
Increase roots surface area
Cant enter crevices and extract water
Carbonic acid produced consequent of root hair respiration helps to break down ions in the soil matrix
Internal Structure of Roots
Root Cap
Interior cells are meristematic
Some cells grow out and some grow around root cap
Cells on the exterior are heavily modified for resilience and only last about 4-5 days before being replaced #
Root Apical Meristem
Root is more orderly than shoot
Quiescent center
Mitotically inactive portion of Apical Meristem
Resistant to various harmful agents- radiation, toxins, etc.
Reserve of healthy cells, can replenish or replace damaged portions of the Root Apical Meristem
Zone of Elongation
Region just behind the apical meristem
Cells expand greatly and elongate here
Outermost cells: Protoderm
Differentiate into Epidermis
Center: Provascular Tissue
Differentiate into primary xylem/phloem
Between protoderm and provascular tissue: Ground Tissue
Differentiates into the Root Cortex
Protoxylem/Protophloem form earliest=closest to meristem
Farther from root tip, older/larger cells develop into metaxylem and metaphloem
Zone of Maturation/Root Hair Zone
Several important processes occur simultaneously
Root hairs grow outward (absorption of H2O and minerals)
Cortex cells continue to enlarge(transfer of minerals from epidermis to vascular tissue)
Apoplastic transport
Symplastic transport
Endodermis
Innermost layer of cortical cells form a cylindrical shape
Radial walls of the endodermis
Encrusted with Lignen and Suberin
Top, Bottom, and Side walls
Casparian Strips #
Waterproof
Acts as a filter for water entering the xylem stream
Impermeable to minerals, protoplasts must intentionally absorb minerals and carry them across Casparian Strip
Many glands and secretory cavities also have Casparian Strips
Vascular tissues
Many tissues of the metaxylem/metaphloem become fully differentiated and functional
Arrangement of vascular tissues is different than that in stems
Xylem as solid mass in center, surrounded by strands of phloem
No pith present
Within the xylem
Inner wide cells=metaxylem
Outer narrow cells= protoxylem (#of strands=#phloem strands)
Phloem
Protophloem occurs on the outer side
Metaphloem occurs on the inner side
Pericycle
Lateral roots initiated here
In between vascular tissue and endodermis
Parenchyma cells
Mature Portions of the Root
Passage cells
Represent passageways for the absorption of materials
Formed by an irregular process of lignen and suberin accumulation
Root Pressure
Caused by absorption of materials in the root hair zone
Extremely important
Origin and Development of Lateral Roots
Lateral roots are initiated by cell divisions in the Pericycle #
Some cells become more densely cytoplasmic with smaller vacuoles and resume mitotic activity
Creating a small root primordium-->Apical Meristem
Apical meristem pushes outward
Root primordium swells into the cortex
Endodermis torn, crushed, or may undergo cell division and form a thin covering over the primordium
Destroys cells of the cortex and epidermis in its way
Lateral root has root cap, first protoxylem/protophloem are connnected to parent root, when it emerges
Other Types of Roots and Root Modifications
Storage Roots
Carbohydrates stored in the root are used to produce a new shoot in the spring
Useful in Plants that are biennial or perennial
Examples
Beets, Carrots, and Celery
Prop Roots
Roots that may grow through the air and into the soil
Act as stabilizers
If roots undergo secondary growth, they can become woody and incredibly strong
Buttress roots
Aerial Roots of Orchids
Many orchids are epiphytic
Roots dangle frely in the air
Root epidermis is composed of several layers of large dead cells and are a waterproof barrier (Velamen)
Contractile Roots
Contractions caused by changes in the shape of cortex cells
Brings the base of the shoot to soil level
Contractile roots are the means by which shoots become anchored in the soil
Mycorrhizae
Symbiotic relationship with fungi
Ectomycorrhizal relationship
Fungal hyphae penetrate between the outermost root cortex cells, but never invade the actual cells
Endomycorrhizal association
Fungal hyphae penetrate the root cortex as far as the endodermis, but not past the Casparian strip
Arbuscule, provides plant with phosphorus
Extremely important, main mechanism of root absorption of phosphorus
Root Nodules and Nitrogen Fixation
Scarcitiy of nitrogenous compounds in soil is a main growth-limiting factor for plants
Nitrogen fixation
Rhizobium has a symbiotic relationship with a small number of plants, especially legumes
Infection thread
Root nodule
Bacteria infects the plant cell cytoplasm, and fills the plant cells with bacteroids capable of converting N2 into nitrogenous compounds that are released to the plant cell