Secondary Growth and Transport in Plants: Structure and Function

Secondary Growth in Plants

Types and Location of Meristems

Meristems are regions of undifferentiated cells in plants that are responsible for growth. There are two main types: apical meristems and lateral meristems. Apical meristems are found at the tips of roots and shoots and are responsible for primary growth (lengthening). Lateral meristems, including the vascular cambium and cork cambium, are responsible for secondary growth (increase in diameter), which occurs in conifers and woody eudicots.

• Apical meristems: Root and shoot tips; all vascular plants

• Lateral meristems: Vascular cambium & cork cambium; only in conifers & woody eudicots

• Secondary growth: Produces wood and bark

Primary vs. Secondary Growth in Stems

Primary growth increases the length of stems and roots, while secondary growth increases their thickness. The vascular cambium and cork cambium are lateral meristems that produce secondary tissues.

• Primary growth: All vascular plants (ferns, seed plants)

• Secondary growth: Woody species (conifers, woody eudicots)

Structure of Secondary Growth

The vascular cambium produces secondary xylem (wood) to the inside and secondary phloem to the outside. The cork cambium produces the periderm, which replaces the epidermis in woody plants.

• Vascular cambium: Adds secondary xylem and phloem

• Cork cambium: Adds secondary dermal tissue (periderm)

Development of Secondary Tissues Over Time

Secondary growth results in the outward expansion of the stem as new layers of xylem, phloem, and periderm are added each year.

• Growth occurs outward from the vascular cambium

• Annual growth rings are formed by the periodic addition of secondary xylem

Vascular Cambium Activity

The vascular cambium produces more secondary xylem than phloem, contributing to the thickening of stems. Cambial initials can divide to form new cambial cells, increasing the circumference of the cambium.

• Most thickening is from secondary xylem

• Cambial initials divide to form new initials and secondary tissues

Wood Anatomy and Growth Rings

Wood Structure: Rays and Growth Rings

Wood consists of secondary xylem, with rays of parenchyma cells for lateral transport. Growth rings are visible in cross-sections and represent annual cycles of growth.

• Rays: Parenchyma cells for lateral transport

• Growth rings: Caused by seasonal changes in xylem cell size and wall thickness

Lignin: Structure and Function

Lignin is a complex organic polymer deposited in cell walls, providing strength and rigidity to wood and bark. It is the second-most abundant natural polymer after cellulose.

• Fills spaces and binds cellulose, hemicellulose, and pectin

• Gives strength to wood and bark

• Used in human products: rope, clothing, paper (lignin removed), artificial vanillin

Bark and Cork Cambium

Bark Structure

Bark includes all tissues outside the vascular cambium: secondary phloem and periderm (cork cambium, cork, phelloderm). The cork cambium produces cork, which protects the plant after the epidermis is lost.

• Cork cambium (phellogen): Arises from cortex cells, produces periderm

• Periderm: Phelloderm (living cells), cork cambium, cork (suberized, dead cells)

• Function: Protection of woody plants

Transport in Plants

Overview of Transport Mechanisms

Transport in plants involves the movement of water, minerals, and sugars through specialized tissues. The movement follows the laws of thermodynamics, with water moving from regions of higher to lower potential energy.

• Xylem: Transports water and minerals upward

• Phloem: Transports sugars and other compounds in any direction

• Mechanisms: Transpiration-cohesion-tension (xylem), pressure-flow (phloem)

Water Potential (Ψ) and Its Components

Water potential (Ψ) determines the direction of water movement. It is measured in megapascals (MPa) and combines solute potential (Ψs) and pressure potential (Ψp).

• Water flows from higher to lower water potential

• Ψ = 0 MPa for pure water at sea level and room temperature

• Equation: $\Psi = \Psi_s + \Psi_p$

Pathways of Water Movement

Water moves through plants via three main pathways: apoplast (cell walls and extracellular spaces), symplast (cytosol connected by plasmodesmata), and transmembrane (across cell membranes).

• Apoplast: Nonliving continuum outside cytosol

• Symplast: Living continuum of cytosol

• Transmembrane: Across cell membranes

Water Potential in Plant Cells

Water movement in plant cells is driven by differences in water potential. Plasmolysis occurs when cells lose water in hypertonic solutions, while turgor pressure develops in hypotonic solutions.

• Water moves from higher to lower Ψ

• Plasmolysis: cell membrane separates from cell wall

• Turgid cell: membrane pushes against cell wall

Xylem Structure and Function

Xylem consists of tracheids and vessel elements, which are dead at maturity and conduct water and minerals upward. The ascent of sap is driven by the transpiration-cohesion-tension mechanism.

• Tracheids: Found in all vascular plants

• Vessel elements: Found in flowering plants

• Mechanism: Water is pulled up by negative pressure (tension) created by transpiration

Phloem Structure and Function

Phloem consists of sieve-tube elements and companion cells. It transports sugars and other organic compounds from sources (e.g., leaves) to sinks (e.g., roots, fruits) via the pressure-flow hypothesis.

• Sieve-tube elements: Main transport cells

• Companion cells: Support sieve-tube elements

• Pressure-flow hypothesis: Movement from source to sink driven by pressure differences

Summary Table: Xylem vs. Phloem

Tissue	Transport Cell Types	What is Transported?	Direction	Mechanism
Xylem	Tracheids, vessel elements	Water & minerals	Upward	Transpiration-cohesion-tension
Phloem	Sieve-tube elements	Sugar water, other compounds	Any	Pressure flow

Additional Info

• Cell walls provide strength due to a complex mixture of polysaccharides

• Tracheids are an example of cells that are dead at maturity but fully functional

• The increase in diameter of woody plants is mostly due to the production of xylem