16 Secondary growth

Definition: The increase in diameter of the stem in regions that no longer undergo elongation.
Causes:
- Activity of two lateral meristems:
- Cork cambium: Forms the protective outer layer.
- Vascular cambium: Forms the xylem and phloem.
Herbaceous plants: Typically undergo very little to no secondary growth.
Woody plants (trees and shrubs): Can undergo hundreds of years of secondary growth.
Function of the vascular cambium:
- Forms secondary
- Xylem: Produced internally.
- Phloem: Produced externally.

Annuals:
- Complete entire life cycle in one growing season.
Biennials:
- Complete life cycle over two growing seasons.
- First season: Develop root system and small rosette of leaves.
- Second season: Plant bolts, flowers, and produces seeds.
Perennials:
- Vegetative structures live multiple years.
- Flower only upon reaching adulthood, which varies among species.
- Many undergo secondary growth.
Life cycle production includes seed to flower, gathering resources throughout.

Composition: Meristematic cells are highly vacuolate (containing large vacuoles).
Two cell forms:
- Fusiform initials: Longer than wide.
- Ray initials: May be elongated or square in shape.
Function after division: Produce secondary xylem and phloem.
- Phloem development occurs externally while xylem develops internally.

Axial system:
- Formed through division of fusiform initials.
- Function: Move water and nutrients up and down within the plant.
Radial system:
- Formed from ray initials.
- Function: Transport and store water and nutrients back and forth between xylem and phloem.
Cambial zone: Includes cambial initials and their immediate derivatives which are not yet differentiated.

In woody stems, much more secondary xylem is produced relative to secondary phloem.
As secondary growth occurs, the primary phloem is pushed outward and crushed.

Periderm development follows initiation of secondary vascular growth.
Function: Replaces ruptured epidermis as protective covering.
Structure: Consists of three layers:
- Cork cambium or phellogen: Meristematic, produces periderm.
- Cork or phellem: Formed exteriorly, serves as protective layer.
- Phelloderm: Resembles cortical parenchyma, forms the interior protective layer.
Composition includes suberized cell walls (containing suberin).

Periderm development typically occurs below the epidermis in the cortical layer in most woody plants.
In some species, it may form deeper in the cortical tissue near the primary phloem.
Cork cells contain suberin or wax, which may lignify, increasing impermeability.
Lenticels: Facilitate gas exchange through the impermeable periderm under the epidermis.

Bark: Refers to all tissue exterior to the vascular cambium, including periderm and cortex.
Composition of periderm: Includes phellogen, phelloderm, and phellem.
Bark characteristics:
- Each growing season, vascular cambium adds secondary phloem and secondary xylem.
- Old secondary phloem tends to be crushed and pushed exterior by the new periderm formation.
Cork and bark are often confused; cork is only one component of the periderm.

Texture of bark determined by periderm formation:
- Scale bark: Forms due to discontinuous periderm growth with overlapping layers.
- Ring bark: Forms from continuous rings of periderm around the stem axis.
- Most often, bark exhibits characteristics intermediate between scale and ring types.

Wood is composed of secondary xylem.
Historical significance: One of the most critical plant tissues for human survival.
Classifications of wood types:
- Hardwood: Derived from angiosperms (specifically magnoliids and eudicots).
- Softwood: Derived from conifers.
- Important note: Classifications do not accurately reflect wood density or strength.

Composition: Simpler than angiosperms.
Characteristics:
- Lack vessels and have less axial parenchyma.
- Consist primarily of long, tapering tracheids.
- In certain species (e.g., pine), parenchyma cells are primarily associated with resin ducts.
- Key differences: Tracheids have narrow ends compared to vessel elements.

Structure: More diverse than conifers.
Composition includes a greater variety of cell types: vessel elements, tracheids, fibers, and parenchyma.
Significance: Provides a more complex appearance compared to softwood.

Formation: Result from the periodic activation of the vascular cambium.
Visibility: Most easily observed in secondary xylem.
Width of growth rings: Dependent on available resources, such as water and nutrients.
Informational value: Can be used to infer historical rainfall data for specific years.
- Early wood: Produced in early growth season, has lower density.
- Late wood: Produced later in the season, characterized by thicker cell walls and narrower cell diameters.

Heartwood: Defined as nonconducting wood that is often impregnated with oils, gums, resins, and tannins.
Sapwood: Still actively conducting water; contains living cells and serves as reserves for nutrients and materials.