Secondary Thickening Flashcards
What is Secondary Thickening?
Process of radial growth in dicot stems and roots, increasing girth
Driven by vascular cambium and cork cambium (lateral meristems)
Where Does Secondary Thickening Occur?
Found in dicotyledons and gymnosperms, absent in most monocots
Occurs in stems & roots, not leaves
What forms the Vascular Cambium?
Forms from procambium and parenchyma cells in vascular bundles
Becomes a continuous ring around the stem
What does the Vascular Cambium produce?
Produces:
* Secondary xylem (inward) → forms wood
* Secondary phloem (outward) → forms inner bark
Cells divide and differentiate based on hormonal signals
What are Growth Rings in Secondary Xylem?
Develop due to seasonal variation:
* Early wood (springwood) → larger cells, rapid growth
* Late wood (summerwood) → smaller, denser cells
Forms majority of a tree trunk
What is the function of Secondary Phloem?
Transports sugars and nutrients via sieve tube elements
Older layers crushed & sloughed off as new phloem forms
What is the Cork Cambium (Phellogen)?
Develops from outer cortex as epidermis splits
Produces periderm, which replaces the epidermis in mature stems
What does Periderm consist of?
Periderm = cork cambium + cork (phellem) + phelloderm
Protects against water loss, pathogens, and mechanical damage
What is the function of Cork (Phellem)?
Dead cells with suberin for waterproofing
Prevents water loss & pathogen entry
What are Lenticels?
Raised, porous structures in bark
Allow gas exchange for underlying living tissues
How does Secondary Thickening affect Structural Integrity?
Strengthens the stem via lignification of secondary xylem
Increases mechanical support for larger plant size
What are the functional differences between Heartwood and Sapwood?
Sapwood = active transport of water
Heartwood = older, non-functional xylem
Often darker due to waste accumulation
What role do Plant Hormones play in Secondary Thickening?
Auxins regulate vascular cambium activity
Gibberellins influence xylem cell expansion
Both are vital for growth regulation
How does Secondary Growth in Roots compare to Stems?
Similar to stems but starts from vascular cambium within the stele
Pericycle contributes to cork cambium formation
What is the importance of Secondary Thickening in Plants?
Increases longevity and structural strength
Enables height growth
Creates wood – a major economic and ecological resource
Supports a larger canopy
What is the primary function of the periderm in plants?
The periderm serves as a protective layer for the plant, replacing the epidermis in woody stems and roots.
True or False: Medullary rays are involved in the transport of water and nutrients in the plant.
False: Medullary rays primarily function in storage and lateral transport of nutrients.
Fill in the blank: The __________ is the central part of a plant stem or root, often used for storage.
pith
What is the role of the cortex in plant structure?
The cortex provides storage and support, and is involved in the transport of substances between the epidermis and vascular bundles.
Multiple choice: Which of the following describes vascular bundles?
C: They are arrangements of xylem and phloem that facilitate the transport of water and nutrients.
