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Concepts Secondary growth in woody plants

Secondary growth, 2 lateral meristems involved Outline: - Concepts - Monocots vs dicots - Xylem development Vascular cambium Cork cambium

2o xylem 2o phloem Periderm = wood

Concepts Concepts •An herb’s conducting capacity is set after a portion of •An herb’s conducting capacity is set after a portion of stem or root is mature stem or root is mature

•Woody plants become wider by accumulation of wood •Woody plants become wider by accumulation of wood and bark, giving them greater conducting capacity and bark, giving them greater conducting capacity •Longevity and size involves advantages and new challenges

Cross sectional area of a ring of wood

Concepts Monocots vs dicots • Monocots stems have neither a vascular cambium • The cambia of gymnosperms and angiosperms nor a cork cambium. are thought to be homologous – Primary xylem and phloem in vascular bundles scattered throughout the stem • True secondary growth occurs in: - Most angiosperm lineages (even in many herbs • Dicot stems have vascular bundles arranged in a there is some degree of woodiness) cylinder. - All gymnosperms - Not in ferns or monocots

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bark Xylem development • Due to xylem expansion, cambium is •cell division pushed outward •cell expansion •secondary cell wall deposition •programmed cell death • Seasonal changes in cambial activity result in growth rings (what about tropical trees?)

• How much xylem is produced relative to phloem?

From Schreiber, Hacke, stem center Hamann, submitted

Development of the vascular cambium Periderm Cork (phellem) • Cork cambium (phellogen) Phelloderm (sometimes) • Lenticels for exchange of gases The vascular cambium is a continuous band, which is formed from the fascicular cambium (in the vascular bundles) and the interfascicular cambium (which forms de novo between vascular bundles).

Lenticel

Cork (phellem)

Cork cambium Photo: L. Plavcova

Periderm

Protection of inner bark and vascular cambium from

– Anything that wants to feed on inner bark – Water loss (cork cells contain suberin, a waxy water impermeable substance) – Frost – Fire

Photo: S. Schreiber

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Lenticels

bochum.de

bochum.de

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Paper birch

Photo: U. Hacke

www.botanik www.botanik Platanus x hispanica Quercus suber Acer griseum Related to American Sycamore Cork Oak Paperbark Maple

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