From Seed to Seed:
Plant Science for K-8 Educators

Types of Meristems

Apical meristems. At the tip, or apex, of each stem and root is an apical meristem. Cell division at the apical meristem contributes to the lengthening of these plant parts. This lengthening is termed primary growth, and it takes place in tender, young tissues. A new seedling reaching for the sky, roots penetrating the soil in search of water-these are examples of primary growth. In addition to lengthening the plant body, the shoot apical meristem perpetually produces leaf and bud primordia, which develop into leaves and lateral shoots, respectively.

Lateral meristems. If apical meristems-and primary growth-were the only means by which a plant grew in size, we'd have very skinny trees! Of course, in addition to lengthening, some roots and shoots also increase in diameter and become woody. This is termed secondary growth, and it takes place in lateral meristems. The two types of lateral meristems in woody plants are found in cylinder-shaped regions below the bark.

One type of lateral meristem is the vascular cambium. The vascular cambium produces the vascular tissue (xylem and phloem) for the plant. The vascular cambium lies between the wood and the inner bark. During secondary growth, new xylem cells formed by the vascular cambium become thick-walled and sturdy, and the living contents die. As the branch or trunk enlarges in circumference, these cells make up the bulk of the stem tissue, forming the secondary xylem, or what we commonly call wood. Thinner-walled phloem cells divide outward from the vascular cambium, surrounding the wood; this forms the inner bark.

If you take a freshly cut branch or log and peel away the bark right down to the wood, you'll feel a moist, slippery surface on both the wood and the inner bark. This is the region of the vascular cambium. Never peel the bark off a living plant!

The cork cambium is a second type of lateral meristem. It lies closer to the surface than the vascular cambium, and produces the outer bark. The outer bark is composed mostly of cork tissue. Cork is a collection of dead cells that replaces the epidermis in woody stems.

Intercalary meristems. In most plants, the primary meristem is located at the tip of the growing shoot-we're all familiar with the rapid growth of a morning glory vine, for example. But consider your lawn. If you cut off the meristem every time you mow the lawn, how does the plant regrow so quickly?

Now we know how plants grow. But plants move and change and react in other ways besides simply growing larger. It is easy to overlook these changes, however, because "plant time" usually moves much more slowly than "human time." Let's take a look at the other ways that plants grow and change.