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 • 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.
The life span of a flowering plant is measured in one-year growing seasons.
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.
During the second growing season, the plant flowers, produces seeds, then dies.
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 • 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.