Secondary growth

• takes place within primary growth after primary tissues are formed
• develops from secondary meristems and enables plant to grow in diameter
• sequence of development:

Vascular cambium

• produces secondary X and P adjacent to primary X and P; develops between primary X and P initially
• as growth in diameter continues, pressure on peripheral tissues causes them to slough off
• first within vascular bundles (fascicular cambium)
• then becomes continuous as interfascicular cambium joins bundles together
• cell types in secondary xylem and phloem same as those in primary xylem and phloem
• appearance different:
• stacked appearance of cells
• presence of vascular rays
• cells of rays also produced by vascular cambium
• oriented radially in secondary growth

• initiates first in cortex as epidermis is disrupted
• produces cork to the outside and phelloderm to the inside; however, all layers become heavily filled with suberin (all layers together called periderm)
• provides a waterproof protective layer as epidermis disappears
• continued growth in diameter causes periderms to fall off; continuously reinitiated in secondary phloem

• gas exchange through corky bumps called lenticels

• in an old woody stem, then, most tissues are secondary:
• wood (secondary xylem)
• bark (secondary phloem and periderms)

• secondary xylem produced in layers that can vary in density depending on water availability
• spring wood: large cells
• summer wood: small cells
• differences in cell density appear as rings ("annual rings" in seasonal environments)
• continuous from roots to stem (includes leaves if they remain on plant)
• softwoods: gymnosperms (contain mainly tracheids, no fibers)
• hardwoods: angiosperms (contain vessels and fibers as well as tracheids)

• transverse (cross)
• longitudinal
• radial: section goes through center along radii; rays seen in side view
• tangential: section does not go through center; rays seen in cross-sectional view

• ring-porous: large vessels produced only in spring; appear to be associated with annual rings (oak)
• diffuse-porous: vessels produced throughout the growing season (maple, Robinia, Tilia)
• nonporous: no vessels (all gymnosperms)
• bordered pits in tracheids
• resin ducts in many species

• older wood sometimes fills with dark-colored deposits (heartwood)

• young, functional wood is lighter in color (sapwood)

• secondary phloem with periderms
• ring-bark: continuous periderms
• bark peels off in strips
• cherry, birch
• scale bark: discontinuous periderms
• bark falls off in small chunks

• oak, hemlock