Biology For Dummies
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Plant cells communicate with each other via messengers called hormones, chemical signals produced by cells that act on target cells to control their growth or development. Plant hormones control many of the plant behaviors you’re used to seeing, such as the ripening of fruit, the growth of shoots upward and roots downward, the growth of plants toward the light, the dropping of leaves in the fall, and the growth and flowering of plants at particular times of the year.

Five categories of hormones control plant growth and development:

  • Auxins stimulate the elongation of cells in the plant stem and phototropism (the growth of plants toward light). If a plant receives equal light on all sides, its stem grows straight. If light is uneven, then auxin moves toward the darker side of the plant. This may seem backward, but when the shady side of the stem grows, the stem, in its crookedness, actually bends toward the light. This action keeps the leaves toward the light so photosynthesis can continue.
  • Gibberellins promote both cell division and cell elongation, causing shoots to elongate so plants can grow taller and leaves can grow bigger. They also signal buds and seeds to begin growing in the spring.
  • Cytokinins stimulate cell division, promote leaf expansion, and slow down the aging of leaves. Florists actually use them to help make cut flowers last longer.
  • Abscisic acid inhibits cell growth and can help prevent water loss by triggering stomates to close. Plant nurseries use abscisic acid to keep plants dormant during shipping.
  • Ethylene stimulates the ripening of fruit and signals deciduous trees to drop their leaves in the fall. Fruit growers use ethylene to partially ripen fruit for sale.

Some of the flavor-making processes that occur in fruits happen while the fruits are still on the plant. So, even though ethylene can trigger some parts of ripening, like softening after a fruit has been picked, fruit that’s picked unripe doesn’t taste as good as fruit that has ripened on the plant. That’s why you can buy a big, beautiful tomato at the grocery store and take it home only to discover that it doesn’t have much flavor — it was probably picked unripe and then treated with ethylene.

If you have houseplants that are growing in bent shapes toward the window, you’re seeing the effect of the hormone auxin at work. The auxin is collecting on the shady side of your plants’ stems, and those cells are growing longer, pushing the stems toward the light. To keep your plants evenly shaped, rotate them occasionally.

If your plants are growing really long and thin, they may not have enough light in the place you put them. If all parts of the stem are too shaded, the auxin will make all sides of the stems grow long and thin. This can make the plants very fragile and they may not have enough light for photosynthesis. If they seem yellowish, that’s another clue.

About This Article

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About the book author:

René Fester Kratz, PhD, teaches biology at Everett Community College. Dr. Kratz holds a PhD in Botany from the University of Washington. She works with other scientists and K?12 teachers to develop science curricula that align with national learning standards and the latest research on human learning.

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