Charles and Francis Darwin found that a grass seedling bent toward light unless its tip was covered. What did this suggest?
AThe tip senses light and sends a signal to the region below that bends
BLight is absorbed only by the roots
CThe bending region senses the light directly
DCovering the tip killed the seedling
In The Power of Movement in Plants (1880) the Darwins showed that bending happens below the tip, yet the tip must be exposed to light. Some signal must travel from the tip downward. Later work, including Frits Went's agar-block experiments in the 1920s, showed that this signal is a chemical, now called auxin.
Question 2 True / False
A shoot lit from one side bends toward the light because the cells on its lit side elongate faster than those on its shaded side.
TTrue
FFalse
Answer: False
The reverse is true. Auxin is moved toward the shaded side. Higher auxin there makes those cells elongate more, so the shaded side grows longer and the shoot curves toward the light.
Question 3 Multiple Choice
Which hormone is a gas that speeds up the ripening of many fruits?
AAuxin
BEthylene
CCytokinin
DGibberellin
Ethylene is a simple hydrocarbon gas. Ripening fruits release it, and it triggers further ripening in nearby fruits. This is why one ripe banana in a closed bag speeds up the ripening of the others.
Question 4 Short Answer
A root placed on its side curves downward. Describe how gravity is sensed and how the curve is produced.
Think about your answer, then reveal below.
Model answer: Cells in the root cap contain dense, starch-filled plastids called statoliths that settle on the lower side of the cell. This leads to more auxin on the lower side of the root. In roots, high auxin slows elongation, so the lower side grows less than the upper side and the root curves downward.
Shoots and roots respond oppositely to the same auxin build-up on the lower side. In shoots it speeds elongation and the shoot bends up; in roots it slows elongation and the root bends down.