单选题

单选题 What do plant hormones do?

A. Regulate the plant's temperature
B. Transport water through the plant
C. Stimulate responses in cells and tissues
D. Make the plant unattractive to animals

【正确答案】 C

【答案解析】[解析] 7-10 The word "hormone" is derived from a Greek verb that means "to excite." Hormones are found in all multi-cellular 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 houseplant on a windowsill 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 nineteenth 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. 7. What do plant hormones do? The professor says: A hormone is a chemical signal," ...it triggers responses in cells and tissues.

单选题 What can be inferred about phototropism in plants?

A. It is similar to phototropism in animals.
B. It relies on hormones produced by the plant.
C. It functions only in the absence of sunlight.
D. It stimulates the production of seeds.

【正确答案】 B

【答案解析】[解析] What can be inferred about phototropism in plants? The professor says: The growth of a plant toward light is called phototropism," 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. You can infer that phototropism relies on hormones produced by the plant.

多选题 Which grass seedlings would probably NOT bend toward light? Click on 2 answers.

Seedling in a moist ecosystem

Seedling with the tip cut off

Seedling wearing a black cap

【正确答案】 B、C

【答案解析】[解析] Which grass seedlings would probably NOT bend toward light? You can infer that a seedling with the tip cut off would not bend toward light: 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. You can also infer that a seedling wearing a black cap would not bend toward light: The seedling wouM also fail to grow toward light if the tip was covered with an opaque cap.

单选题 What can be inferred about the tip of a plant's stem?

A. It stops growing once the plant produces real leaves.
B. It plays an important role in temperature control.
C. It can be removed with no influence on the plant.
D. It produces a hormone that affects the stem's growth.

【正确答案】 D

【答案解析】[解析] What can be inferred about the tip of a plant's stem? The professor says: The Darwinsproposed the hypothesis' that some signal was transmitted downward from the tip into the part of the stem that controlled growth; These chemical messengers were hormones. You can infer that the tip of a plant's stem produces a hormone that affects the stem's growth.