Rucete ✏ Campbell Biology In a Nutshell
Unit 6 PLANT FORM AND FUNCTION — Concept 35.4 Secondary Growth Increases the Diameter of Stems and Roots in Woody Plants
Secondary growth allows woody plants to increase in thickness over time, producing structures like wood and bark. This process is driven by two lateral meristems: the vascular cambium and cork cambium, enabling trees to grow tall, sturdy, and protected.
What Is Secondary Growth?
- Occurs in stems and roots, not leaves
- Adds thickness (girth) via:
- Vascular cambium: adds secondary xylem (wood) inward and secondary phloem outward
- Cork cambium: adds cork, forming the periderm
- Common in gymnosperms and eudicots, rare in monocots
- Occurs in older parts of plants after primary growth ceases
The Vascular Cambium
- Ring of meristematic cells between primary xylem and phloem
- Produces:
- Secondary xylem: forms most of the wood
- Secondary phloem: transports sugars; older phloem is sloughed off
- Vascular rays: radial files of parenchyma for storage and repair
Wood and Growth Rings
- Early wood (spring): large-diameter, thin-walled xylem for water transport
- Late wood (summer): small, thick-walled cells for strength
- Growth rings:
- Used in dendrochronology (tree-ring dating)
- Thicker rings: favorable conditions (warm/wet)
- Thinner rings: dry or cold years
- Heartwood: older, non-conductive, resin-filled for support
- Sapwood: outer xylem still active in water transport
The Cork Cambium and Periderm
- Epidermis ruptures and is replaced by periderm as stems thicken
- Cork cambium in cortex/phloem produces cork cells:
- Dead at maturity, coated with suberin for waterproofing
- Forms barrier against water loss and infection
- Lenticels: openings in periderm for gas exchange
- Bark includes:
- All tissues external to vascular cambium: secondary phloem, cork cambium, cork, and periderm
- Outer bark layers slough off with age
Evolution and Adaptation of Secondary Growth
- In Arabidopsis, added mechanical stress can trigger wood formation
- Genes for primary growth are involved in secondary growth
- Suggests shared evolutionary origin for both growth types
In a Nutshell
Secondary growth thickens stems and roots, building wood and bark that support and protect the plant. The vascular cambium creates secondary xylem and phloem, while the cork cambium forms cork for waterproofing and defense. Tree rings record seasonal changes, and even hollow trees can survive due to active sapwood. Together, these tissues help long-lived plants grow strong and resilient.