Formation of Periderm
Pericycle cells divide periclinally into two layers.
Phellogen originates from the outer sister cells of pericycle. The inner sister cells give rise to the secondary cortex. The epidermis and cortex die and are sloughed off.
Cortex
Endodermis
The innermost layer, compactly arranged, contain Casparian strips on anticlinal walls. Radial conduction by way of the protoplasts.
Vascular Cylinder
The Secondary Structure
Originates from secondary meristem (phellogen and vascular cambium) Occurs in gymnosperms and most dicots.
Two parts: Periderm: from phellogen (cork cambium) Secondary Vascular Tissues: from vascular cambium
Cortex
Storage Parenchyma Cells
Parenchyma cells with numerous intercellular spaces and plasmodesmata. Storage of starch
Radial conduction by way of the protoplasts and cell walls.
Cortex
Hypodermis
The outermost layer of packed parenchyma cells which resemble epidermis. Sometimes with suberized cell walls (beyond the root hair zone). Retain water and nutrients
Vascular Cylinder
Primary Xylem
Forms radiating ridges (Diarch, Triarch, Tetrarch, Polyarch) Two parts: protoxylem: differentiates early, while adjacent cells are still elongating. Metaxylem: differentiates late, after adjacent cells have completed their elongation. Exarch: primary xylem differentiates from outside to inside.
Formation of Secondary Vascular Tissues
Vascular cambium originates from part of pericycle cells and procambial cells that remain between primary xylem and primary phloem Secondary xylem and phloem are added to the root and vascular rays are produced Finally, the primary phloem is crushed but fibers may remain.
The Primary Structure
Epinder
Epidermis
Formed by the protoderm Closely packed parenchyma cells with a large numbers of root hairs (tubular extensions of the epidermal cells) Absorbing surface.
Pericycle
Primary Xylem
Primary Phloem Pith
Vascular Cylinder
Pericycle
A meristematic layer of parenchyma cells one to several cells thick. Contributes to the cork cambium and vascular cambium Produces lateral roots (endogenous).
Cortex
Formed by the ground meristem.
Composed of parenchyma cells which occupy the largest cross-sectional area of a primary root. Consists of three concentric layers: Hypodermis Storage Parenchyma Cells Endodermis
Tertiary Structures
Concentric layers of extra cambia
Auxillary stele
Interxylary cork
Vascular Cylinder
Primary Phloem
Nestled strands between the ridges of xylem Two parts: Protophloem Metaphloem Exarch
Vascular Cylinder
Pith
Parenchyma tissue in the center, located interior to the vascular bundles In root, pith mainly occurs in monocots and a few dicots.
Overall View of Secondary Structure
Remnants of the epidermis and cortex or none Periderm Pericycle (secondary cortex) Primary phloem (fibers and crushed cells) Secondary phloem Vascular Vascular cambium rays Secondary xylem Primary xylem