# PTE Listening MCM #8: Phototropism in Grass...

Listening: Multiple Choice, Multiple Answers #8 - Level 3 - Feeds Listening

> Listen to the recording and answer the question by choosing all the options that are correct. More than one answer is correct, and a wrong choice costs a point.

Which grass seedlings would probably NOT bend toward light?

- Seedling in a moist ecosystem
- Seedling wearing a black cap
- Seedling with multiple shoots
- Seedling with the tip cut off

## Answer

- Seedling wearing a black cap
- Seedling with the tip cut off

## Explanation

The correct choices are those where the light-sensing mechanism is either physically removed or blocked from detecting the light source, preventing the hormonal signaling required for growth curvature.

## Hints

- Consider where the plant detects light and how it communicates that information to the rest of the stem.
- Think about what happens if the part of the plant that 'sees' the light is covered or missing.

## Transcript

The word hormone is derived from a Greek verb that means to excite. Hormones are found in all multicellular organisms and function to coordinate the parts of the organism. A hormone is a chemical signal. It's produced by one part of the body and is then transported to other parts of the body, where it triggers responses in cells and tissues. The concept of chemical messengers in plants first emerged from a series of classic experiments on how plant stems respond to light. Think about this. A house plant on a window sill grows toward light. If you rotate the plant, it will soon reorient its growth until its leaves again face the window. The growth of a plant toward light is called phototropism. In a forest or other natural ecosystem where plants may be crowded, phototropism directs growing seedlings toward the sunlight that powers photosynthesis. Some of the earliest experiments on phototropism were conducted in the late 19th century by Charles Darwin and his son, Francis. The Darwins observed that a grass seedling could bend toward light only if the tip of the shoot was present. If the tip was removed, the shoot would not curve toward light. The seedling would also fail to grow toward light if the tip was covered with an opaque cap. The Darwins proposed the hypothesis that some signal was transmitted downward from the tip into the part of the stem that controlled growth. Later experiments by other scientists studying phototropism led to the discovery of chemical messengers that stimulated growth in the stem. These chemical messengers were hormones.

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