Biology Semester II

Sections:

Introduction |  Section 1 | Section 2 | Section 3

Section One:

Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6 | Part 7 | Part 8

Biology: Plant Organs, Tissues, and Cells: Part Two Plant Organs: Stems, Roots, Leaves, and Flowers The stem is perhaps the most ancient of the plant organs. The earliest plants, found in rocks dated at over 400 million years old, are little more than naked stems: no roots, no leaves, etc. The stem can conduct photosynthesis in its cells, as in modern cactus, palo verde, and other desert plants. Saguaro cactus and palo verde trees carry out photosynthesis in their stems. The stem produces the reproductive structures of the plant (such as cones, flowers, or sporangia that contain the spores). The stem also produces leaves. Many botanists consider that leaves evolved as tissue developed between flattened, branching parts of stems. In plants that have leaves, the stem serves as a support, raising the leaves above ground to get them into sunlight. The stem also serves a transport function, with water and minerals moving upward and sugar and hormones moving downward through the stem. Flowers and leaves. Stems have specialized areas at their tips where new cells are produced. These areas are called meristems. Stems also have lateral (or axillary) buds, which are embryonic meristems in an arrested state of development. You are eating plant stems when you eat asparagus. Some stems, such as those of red and Irish potatoes, are underground and are modified for storage. As anyone who has “lost” a potato in the refrigerator for a few months can testify, the potato eyes are the buds from which new stems will grow. Buds on a stem. The bud at the tip is the terminal bud; the ones lower on the stem are axillary buds. The root serves a number of important functions in the plant: storage, anchorage, absorption of water and minerals, and vegetative propagation (asexual reproduction). Many plants, such as sugar beets and carrots, store excess sugars and other chemicals in their roots. We, of course, eat these root vegetables. Sugar is not produced in the root. Rather, it is made in the stems and leaves by photosynthesis. Sugars are transported to the root and stored as starch. If you have ever tried uprooting a tree or weed (and the soil was not moist), you can attest to the ability of roots to hold a plant firmly in the soil. Roots absorb water and minerals from the soil and pass those materials into the water-conducting xylem cells for transport to other parts of the plant. Roots also may serve in the asexual (or vegetative) propagation of the plant. Aspen trees commonly will send out roots that poke to the surface some distance from the plant and sprout a new tree. Grass-like plants will propagate in a similar way. The Japanese plant kudzu was imported to the American south many decades ago. It has become a pest, overgrowing buildings, agricultural fields, and power lines. Kudzu can regenerate from a small piece of root. This makes its eradication all but impossible. Mangrove prop roots (L), and roots exposed by erosion (R). Now go on to the next part.

© 2009 Aventa Learning. All rights reserved.