Secondary Growth

• Secondary growth, also known as lateral growth or growth in diameter, occurs in the stems and roots of "woody" plants
• Herbaceous plants do not undergo secondary growth and are often annuals, completing their life cycle from seed germination to seed set in one growing season
• Biennial plants take two growing seasons to go from seed to seed and may exhibit secondary growth in roots, and sometimes in stems
• Woody plants are usually perennials, with vegetative structures surviving for more than two years, flowering at maturity, and exhibiting secondary growth in roots and stems
• Secondary growth involves meristems, which are discrete regions of cell division, and is driven by lateral meristems that produce growth in diameter
• Periclinal divisions of meristematic cells occur parallel to the surface, while anticlinal divisions occur perpendicular to the surface during secondary growth
• In stems, secondary growth involves the formation of fascicular cambium from procambial cells in the vascular bundle, and interfascicular cambium from parenchyma cells between vascular bundles
• The combination of fascicular cambium and interfascicular cambium forms the vascular cambium in stems during secondary growth
• Vascular cambium, a sheath of meristematic cells in stems, produces secondary xylem and secondary phloem through division
• As the stem increases in diameter, cells of the vascular cambium must occasionally divide anticlinally to support growth during secondary growth
• Two types of meristematic initial cells in the vascular cambium are fusiform initials, which are long and tapered, producing secondary xylem and phloem, and ray initials, which are short and cuboidal, producing parenchyma for radial transfer
• The arrangement of fusiform and ray initials contributes to the "figure" in wood, visible in tangential sections
• Annual rings in stems show variation in the size or wall thickness of xylem cells over the growing season, with different characteristics in conifers and ring porous angiosperms
• Knots in stems are branches buried within the growing stem, categorized as tight knots or loose knots
• Sapwood in stems contains living cells, while heartwood consists of all dead cells, often dark-colored due to tannins, oils, resins, and gums
• Hardwoods mainly come from flowering trees and have a variety of cell types, while softwoods are from conifers and have uniform cells, mainly tracheids
• Secondary growth in roots starts in the zone of maturation, where procambial cells and pericycle cells become meristematic, leading to the formation of vascular cambium
• Roots and stems with secondary growth are protected by the periderm, which is the dermal tissue of the secondary plant body
• Periderm consists of cork, phellogen, and phelloderm, with cork cambium dividing periclinally to produce cork cells encrusted with suberin and wax
• Bark, which includes secondary phloem and periderm, forms outside the vascular cambium and contains lenticels for gas exchange through the bark
• The pattern and time of new cork cambium formation, lenticels, and phloem fibers result in distinctive bark characteristics in different plant species