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14.4 Plant Growth. The Role of Plant Hormones. Hormones play a part in regulating a plant's growth. One example is gibberellin (gihb ur EHL ihn). About a hundred years ago, Japanese rice farmers observed that some of their rice seedlings grew abnormally tall.

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14 4 plant growth

14.4 Plant Growth

The Role of Plant Hormones


Hormones play a part in regulating a plant's growth.

  • One example is gibberellin (gihb ur EHL ihn).

About a hundred years ago, Japanese rice farmers observed that some of their rice seedlings grew abnormally tall.

  • This was the first clue to the effects of gibberellin.

Gibberellin can make dwarf plants grow tall, flowers produce seedless fruits, and plants flower prematurely.

  • Young leaves, roots,and plant embryos make gibberellin.

Gibberellin can have an effect on stem length.

  • The hormone also plays a role in seed germination and the growth of new leaves, branches, fruits, and flowers.

These effects make gibberellin useful in agriculture.

  • For example, it is used on seedless grapes to make the fruit grow larger.

Plants in a window usually bend toward the light.

  • You may wonder how the plant is attracted to the light.

Turning in response to an environmental stimulus, such as light or gravity, is called tropism (TROH PIHZ uhm).

  • When a plant turns toward light, it is called phototropism.

Tropisms are controlled by a plant hormone called auxin (AWK sihn).

  • Auxin is produced at the growing tips of plants.

Experiments show that auxin is sensitive to light.

  • As a result, auxin concentrations are always higher on the shaded side of a stem.

Auxin causes the stem cells on the shaded side to grow longer, making the stem bend toward the light.

seasonal responses
Seasonal Responses
  • Why do some plants always flower in summer and not at any other time of year?

Plants have a light sensitive chemical that helps them measure the length of periods of darkness and light.


Summer-blooming plants, such as columbines, are long-day plants.

  • They need long days and short nights to trigger flowering.

Autumn-blooming plants, such as chrysanthemums, are short-day plants.

  • They flower when there are short days and long nights.

Broad-leaf trees also respond to seasonal changes in day length.

  • As days become shorter in autumn, the green chlorophyll in leaves is destroyed and other colors, such as reds and yellows, can then show through.

Some plants, called annuals, complete their entire life cycle within one growing season.

  • After setting seed, the plant dies.

During the first growing season, the plant germinates and develops roots, stems, and leaves.

  • It stores energy in underground organs.

Other plants, called perennials (pur EHN ee uhls), live for many growing seasons.

  • Most develop thick, woody stems and reproduce over and over again.
check and explain
Check and Explain
  • 1. How is plant growth different from animal growth?
  • 2. Name two plant hormones. Describe where each one is produced and its effects on plant growth.
  • 3. Compare and Contrast What do plants that bloom in the summer have in common? How do they differ from plants that bloom in the autumn?
  • 4. Hypothesize Write hypotheses you could use to investigate whether an unfamiliar plant was an annual, a biennial, or a perennial.