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Factors affecting the Direction of Growth of Roots, Stems, and Leaves in a Dicot Plant

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Author: Janice VanCleave

All parts of a plant grow. Plant stems and roots elongate in predetermined directions (up and down, respectively). The stems and roots also increase in diameter, and leaves increase in surface area.

In this project, you will have the opportunity to observe the direction of root, stem, and leaf growth. You will also determine the location of any new growth of these three plant organs. The growing point of these three plant organs will be determined. You will also look at factors affecting growth, such as temperature, light, and plant types.

Getting Started

Purpose: To observe the elongation of a dicot (plant that has seeds with two seed leaves) plant stem and to determine whether the entire stem of the plant is vegetative.

Materials

  • 3 7-ounce (210-ml) paper cups
  • potting soil
  • ruler
  • pencil
  • 12 pole pinto beans
  • water
  • marking pen

Procedure

  1. Fill each paper cup with potting soil to within 1 inch (2.5 cm) from the top.
  2. Use the pencil to punch four to six holes near the bottom of each cup.
  3. Lay four beans on the surface of the soil in each cup.
  4. Cover the beans with about 1 inch (2.5 cm) of soil.
  5. Moisten the soil with water.
  6. Allow the seeds to germinate and the seedlings to grow to a height of about 6 inches (15 cm) above the top of each cup. Note: This will take from five to seven days.
  7. Use the marking pen to mark a line across the stem of each plant level with the lip of the cup. Use this line as the starting line.
  8. Mark three equal sections on each stem between the cotyledons (part of the seed that contains stored food for the plant embryo), or seed leaves, and the starting line (see Figure 11.1).
  9. Mark three equal sections on each stem between the cotyledons and the true leaves at the top of the stem.
  10. Prepare drawings of each plant, numbering each section from 1 to 6, starting at the top of each stem as in Figure 11.1.
  11. Measure and record the length of each section.
  12. At the same time each day for seven days, measure and record the length of each section again.

Results

The author's plants showed the greatest elongation in the sections marked 1, only slight changes in the sections marked 2, and no measurable changes in the lower sections marked 3 through 6.

Why?

Elongation of plant stems occurs primarily in the meristems. Meristem is a term derived from the Greek word meristos, which means "divided." The apical part of the stem (near the tip) has the meristematic cells (cells that divide), and it is at the tip where the growth of the stem occurs.

Cell division is carried out by a process called mitosis. In mitosis, the cell parts duplicate themselves and then divide into two separate cells. This division and duplication process occurs in meristematic cells. As the stem grows, small masses of meristematic cells are left behind, and it is at these points that branches and leaves develop.

Try New Approaches

  1. Does the rate of meristematic growth change during different periods of the day? Repeat the experiment using only the sections of the stems above the cotyledons. Measure these sections at two-hour intervals during the day, starting early and stopping as late as possible. Prepare a data table of the results similar to the one shown on page 72.
  2. Direction of Growth of Roots, Stems, and Leaves

  3. Does light duration affect the rate of meristematic growth? Photoperiodism is the development of an organism depending on the duration of daylight or darkness. Test the effect of light or its absence by repeating the original experiment using three test groups of plants. Make every effort to vary only the amount of light that each group receives. Place one group near a window to receive sunlight, the second group under artificial lighting 24 hours a day, and the third group in a dark closet. Measure the plants daily.
  4. Does the length of the stem tip actually involved in the growing process vary in different plants? Repeat the original experiment using seedlings from different plants. Divide the stem section above the cotyledon leaves into six parts to make a better comparison of the length of the growing area on the tips of each type of plant. Science Fair Hint: Use diagrams showing the changes in the length of each section as part of a project display.
  5. Does temperature affect the growth of meristematic cells? Repeat the original experiment to prepare three plants for study. Design a method so that the growing conditions (water, light, nutrients, and so on) for each plant are the same except for temperature. You could make miniature plastic greenhouses using ice cubes to cool one greenhouse, direct sun to warm another, and a shaded area that gets indirect sun for the third.
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