Xylem and Phloem

The xylem is the principal water-conducting tissue of vascular plants. It consists of tracheary elements, tracheids and wood vessels and of additional xylem fibres. All of them are elongated cells with secondary cell walls that lack protoplasts at maturity. Bordered pits are typical for tracheids, while wood vessels are marked by perforated or completely dissolved final walls.

Phloem is the vascular tissue plants use to transfer sugars from sites of production or storage to locations where energy is needed. In contrast to xylem, phloem tissues are living. Sieve tube members connect at specialized areas called seive plates. At maturity the seive tube members lose their nuclei and fill with a complex proteinaceous material called cell sap. Sugars are transported through the cell saps of adjacent cells.

In losing their nuclei, sieve tube members lack the molecular control mechanisms most living cells possess. Nucleated cells adjacent to sieve tube members appear to take over the control of cellular functions within these phloem transport cells. These nucleated cells are appropriately called companion cells.

Sieve plates are specialized areas where materials flow through tiny pores from one sieve tube member to another.
Most phloem cells are parenchyma cell types. Sometimes phloem tissues contain sclerenchyma fibers for support.

External and Internal Parts of Plant Organs

Plant Hormones

Auxins promote stem elongation, inhibit growth of lateral buds (maintains apical dominance). They are produced in the stem, buds, and root tips. Example: Indole Acetic Acid (IA). Auxin is a plant hormone produced in the stem tip that promotes cell elongation. Auxin moves to the darker side of the plant, causing the cells there to grow larger than corresponding cells on the lighter side of the plant. This produces a curving of the plant stem tip toward the light, a plant movement known as phototropism.

Auxin also plays a role in maintaining apical dominance. Most plants have lateral (sometimes called axillary) buds located at nodes (where leaves attach to the stem). Buds are embryonic meristems maintained in a dormant state. Auxin maintains this dormancy. As long as sufficient auxin is produced by the apical meristem, the lateral buds remain dormant. If the apex of the shoot is removed (by a browsing animal or a scientist), the auxin is no longer produced. This will cause the lateral buds to break their dormancy and begin to grow. In effect, the plant becomes bushier. When a gardener trims a hedge, they are applying apical dominance.

Gibberellins promote stem elongation. They are not produced in stem tip. Gibberellic acid was the first of this class of hormone to be discovered.

Cytokinins promote cell division. They are produced in growing areas, such as meristems at tip of the shoot.

Abscisic Acid promotes seed dormancy by inhibiting cell growth. It is also involved in opening and closing of stomata as leaves wilt.

Ethylene is a gas produced by ripe fruits. Why does one bad apple spoil the whole bunch? Ethylene is used to ripen crops at the same time. Sprayed on a field it will cause all fruits to ripen at the same time so they can be harvested.

Dark Reaction of Photosynthesis: Puzzle

Photosynthesis word search game » word search puzzle maker