Chapter 23

Secondary Growth

Most monocots and certain herbacious dicots cease growth when primary tissues mature.

In contrast, gymnosperms and woody dicots continue growth in diameter even after growth in height (primary growth) has stopped or slowed down.

This increase in diameter in regions that have stopped elongating is called secondary growth. It results in encreased thickness or girth of the plant body.

Secondary growth is due to the activity of two lateral meristems (as opposed to apical meristems). These are:

• Vascular Cambium: secondary phloem and xylem
• Cork Cambium: (cork) - layer of secondary tissue just below the epidermis

Herbs have no secondary growth. Woody plants have primary growth followed by secondary growth during one or more growing seasons.

• Growing Season: a few weeks (among weeds) to a year; the time of year when the plant grows.
• Annuals: weeds, wildflowers, etc.; complete entire life cycle from seed to plant to seed in one growing season. Most annuals have a growing season of less than one year. They usually have no secondary growth.
• Biennials: life cycle takes two growing seasons (2 years) to complete. Biennials usually have no secondary growth.
• Perennials: vegetative structures live or are replaced year after year.
  • Herbaceous Perennials: survive winter by dormant underground roots, or under ground stems (rhizomes, bulbs, or tubers).
  • Woody Perennials: survive above and below ground, but become dormant.
• Vascular Cambium: secondary meristematic cells that produce secondary xylem to the inside and secondary phloem to the outside.
• Secondary xylem, the wood, is highly lignified (at maturity). It is composed of sclerenchyma cells.
  • Early, or spring wood: secondary xylem composed of collenchyma cells; light color
  • Late, or summer wood: the darker colored, denser, thick, lignified walls.

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