Secondary Growth in Woody Plants

Seasonal growth cycles proceed from seed germination, to seed formation, to death.

Life cycles of flowering plants can be of three types:

Secondary growth occurs in all gymnosperms, some monocots, and many dicots. A ring of vascular cambium produces secondary xylem and phloem. Wood is the accumulation of these secondary tissues, especially xylem. Vascular cambium is a cylinder like lateral meristem. It produces secondary xylem on its inner face and secondary phloem on its outer.

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

REVIEW: In early wood, cells have _____ diameters, and _____ walls.

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