From Seed to Seed:
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.
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.
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?In prairie habitats, plants are continually exposed to grazing by deer, bison, and other herbivores. Plants with apical meristems would repeatedly lose their growing points and therefore their primary means of regenerating. Grasses, however, adapted by locating their meristem tissue at nodes along the stem, just above ground level. Grasses continue to grow from these intercalary meristems and can repeatedly renew themselves after grazing by animals (or mowing by humans!)
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.
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Copyright 2001, National Gardening
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