Plants, like animals, exhibit regulatory mechanisms in order to maintain homeostasis. Charles Darwin studied the regulatory mechanisms of thigmotropism (growth in response to touch) and phototropism (growth in response to light) in plants. Darwin concluded that the bending of plants towards a stimulus was due to the alternate elongation of cells within the stem. The collective works of Charles and Francis Darwin, Peter Boysen-Jensen, Paul and F. W. Went led to the discovery of auxin, a growth-promoting plant hormone that is involved in phototropism.

phototropism and coleoptile

- In 1880, Charles Darwin and his son Francis studied phototropism in plants. They observed the growth of a grass seedling under different conditions as seen in the diagram to the left. - Notice that the coleoptile (tip) grows toward the light under normal conditions, when the tip is covered with a transparent sheath and when the base is covered with an opaque sheath. However, when the tip is cut off or covered with an opaque sheath, the grass seedling grows straight up. - From their results, Darwin and his son concluded that there was some signal transmitted from the tip of the seedling that caused the plant to grow towards the light.

- Drawing on the work of Darwin and Darwin, Peter Boysen-Jensen and Paul observed the growth of the coleoptile with the tip separated by mica, a mineral block, and with the tip separated by agar, a gelatinous material. - Notice from the diagram that the seedling grew normally when the tip was separated with agar, but did not grow towards the light when separated with mica. - Boysen-Jensen and Paul concluded that there was some chemical that was being transmitted from the tip to the stem. The chemical was not able to pass through a solid material such as mica, but was able to pass through a liquid permeable barrier, such as agar.

- In 1926, Fritz Went continued studying phototropism in plants. - Went removed the tip of a grass seedling and placed it onto a block of agar. He then placed the agar block in different positions on the tips of decapitated seedlings. When he placed it in the center of the seedling tip, it caused the seedling to grow straight up. When he placed the agar block off-center on the tip, it caused the side of the tip to grow, causing a bending. - Went concluded that the chemical from the tip caused growth in the coleoptile. Went named this growth hormone auxin.

Which one of the following would not provide evidence that the tip of the plant signals the direction in which the plant grows?

- Plants with their tips covered with an opaque sheath grow towards the light.

Which one of the following is the best reason why it is thought that plants, like animals, exhibit behavior?

- They respond to environmental factors.

Which one of the following represents the correct sequence of scientists in the discovery of the plant growth hormone, auxin?

- Charles Darwin & son; Boysen-Jensen & Paul; Fritz Went

Why did Boysen-Jensen and Paul use mica to separate the plant tips from the base of the plants?

- They wanted to block any chemicals that may come down from the tip of the plant.

In what way does phototropism contribute to homeostasis in plants?

- It insures that the shoots of the plant get the maximum amount of light.

13.2.1 Regulation in Plants Flashcards by Irina Soloshenko

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13.2.1 Regulation in Plants Flashcards

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