Secondary Growth in Woody Plants

Life cycles of flowering plants can be of three types:

• Annuals such as corn live one season and there is little secondary growth.
• Biennials, such as carrots, live two seasons. Vegetative growth occurs in the first season, and flower and seed formation in the second.
• Perennials live many years, produce seeds year after year, and have secondary growth. Some examples are shrubs, vines, and trees.

The vascular cambium or lateral meristem is one or two cells thick. It divides in two directions. Cells formed to the outside become secondary phloem. Cells formed to the inside become secondary xylem. The mass of xylem crushes the phloem cells (from last season), which are replaced outside the growing core of xylem.

In response to rupture of the outer cortex (by girth expansion), cork cambium produces the periderm which is a corky replacement of the epidermis.

Tree rings are concentric rings of secondary xylem and indicate annual growth layers. In regions with cool winters or dry spells, the vascular cambium is inactive part of the year. Early wood (beginning of the growing season) contains xylem with large diameters and thin walls. Late wood contains xylem with small diameters and thick walls. Tree rings appear as alternating light bands of early wood and dark bands of late wood, defining the annual growth layers.

Hardwoods (such as oak and hickory) are dicots and have tracheids, vessels, and fibers in their xylem. Softwoods (such as pine and redwood) are gymnosperms. The xylem is composed mostly of tracheids and there are no vessels or fibers. Softwoods typically grow more rapidly than hardwoods.

This animation (Audio - Important) describes secondary growth.

REVIEW: Older roots and stems thicken through activity at ____ .
a. apical meristems
b. cork cambium
c. vascular cambium
d. both b and c
Answer d