Secondary Growth and Wood Anatomy

Secondary growth thickens stems and roots by adding wood and bark from two lateral meristems, allowing trees to build the durable, water-conducting trunks that dominate forests and record their history in annual rings.

Definition

Secondary growth is the increase in girth produced by lateral meristems, and wood anatomy is the study of the secondary xylem they generate, including its cell types and growth-ring structure.

Scope

This topic covers the vascular cambium and cork cambium, the production of secondary xylem (wood) and secondary phloem and periderm (bark), the structure of softwoods and hardwoods, and the formation and interpretation of growth rings.

Core questions

• How do the vascular cambium and cork cambium add girth to stems and roots?
• What distinguishes the wood (secondary xylem) of conifers from that of flowering plants?
• How do annual growth rings form, and what do they record about a tree's life and climate?

Key theories

Lateral meristems and girth

The vascular cambium produces secondary xylem inward and secondary phloem outward, while the cork cambium produces protective periderm, so girth increases as these cylinders of dividing cells lay down new tissue.

Growth rings as a record

Seasonal variation in cambial activity produces alternating earlywood and latewood, forming annual rings whose widths archive a tree's age and the environmental conditions it experienced.

Mechanisms

The vascular cambium is a cylinder of dividing initials that cuts off secondary xylem cells toward the stem center and secondary phloem cells toward the outside; lignification of xylem cell walls produces wood. In temperate climates, large thin-walled vessels or tracheids form in spring (earlywood) and smaller thick-walled cells later in the season (latewood), producing the visible boundary of an annual ring. The cork cambium meanwhile generates suberized periderm that replaces the epidermis as the bark.

Clinical relevance

Wood anatomy underlies forestry, timber engineering, and paper production, while the dating and climate signals in growth rings (dendrochronology) support archaeology and the reconstruction of past climates.

History

Detailed description of cambial activity and wood structure matured in twentieth-century anatomy, while Douglass's founding of dendrochronology showed that growth rings could date wooden artifacts and reconstruct climate.

Key figures

• Katherine Esau
• Andrew Ellicott Douglass

Seminal works

• evert2006
• raven2013

Frequently asked questions

Why do trees form annual rings?

In seasonal climates the cambium produces wide, thin-walled cells in spring and denser, thick-walled cells later in the year; the abrupt transition between one year's latewood and the next year's earlywood appears as a ring boundary.

What is the difference between hardwood and softwood?

Softwood is the wood of conifers, built mainly of tracheids, whereas hardwood is the wood of flowering plants and contains vessels in addition to fibers and tracheids; the terms describe botanical groups, not literal hardness.