All of the cells and tissues that are found in the body of an herbaceous plant are produced by apical meristems located at the growing tips of the stems and roots; such tissues are called primary tissues. In woody plants, most of the tissues of the plant body are produced not by apical meristems, but by specialized regions of cell division and growth located elsewhere and known as lateral meristems. Tissues produced by lateral meristems are termed secondary tissues, and the increase in plant size that they cause is termed secondary growth. In the first part of this week's lab, you will examine the internal structure of a woody dicot twig to help you understand the phenomenon of secondary growth.

Using the photographs below, you should be able to identify the following features associated with secondary growth of the stem:

Cortex. A well-developed cortex composed of parenchyma cells can usually be seen in young woody stems.

Vascular Cambium. The lateral meristem responsible for producing the secondary vascular tissues is known as the vascular cambium. Cell divisions in this meristematic region produce the cells that differentiate into secondary xylem and secondary phloem.

Secondary Xylem. The cells produced by the vascular cambium towards the inner side of the stem differentiate into xylem. Since these cells are produced by a lateral meristem, the tissue is referred to as secondary xylem. Like the primary xylem of an herbaceous plant, secondary xylem is composed of parenchyma cells, sclerenchyma fibers, tracheids, and vessel elements. Secondary xylem is what we commonly refer to as the wood of a plant.

Annual Rings. Each year, the vascular cambium produces new layers of secondary xylem and secondary phloem. These layers are more conspicuous in the secondary xylem, where they are recognizable as the annual rings in the wood. The age of a woody plant can be easily determined by counting the annual rings in a cross section of the stem.

Secondary Phloem. The cells produced toward the outside of the vascular cambium differentiate into secondary phloem tissue composed of parenchyma cells, sclerenchyma fibers, sieve tube members, and companion cells.

Bark. The epidermis forms the protective outer layer of herbaceous stems and of very young woody stems. Eventually, however, in woody stems a second lateral meristem, known as the cork cambium, develops in the cortex. Cork cells, produced by this cambium, combine with old, dead, secondary phloem cells to form the bark of the plant. You can see the beginning of bark formation in the left-hand photograph below:
 

Low magnification view of a young, woody basswood twig in cross section showing the vascular cambium secondary xylem and phloem, and the developing bark.	Higher magnification view of the vascular cambium and secondary vascular tissues. The secondary phloem consists of alternating bands of sclerenchyma fibers and transport cells.

This page last updated: 06 Oct 1999