New research shows that trees appear to sleep when night falls. Scientists used lasers to measure the changes in the positions of birch trees between day and night in Finland and Austria. They found out that after the sun sets, the leaves and branches sink up to 10 centimeters—almost as if they were sleeping. The scientists conducted the tests under dry, windless conditions to eliminate other potential factors. Why did the trees droop? One theory is that water pressure inside the tree tissue is lower overnight because the trees’ cells are not busy photosynthesizing. When the pressure is lower, plants are not as rigid, so they droop. The pressure change might also be because the tree redistributes starches and sugars as it gets ready for the next day. Another reason could be that the trees really are resting. At night, leaves and branches don’t need to reach up to the sun, so the tree can save energy by relaxing them.
【題組】76. Why did the scientists do the tests in certain conditions?
(A) They wanted to ensure the tests were not influenced.
(B) They wanted to measure how long it took for the sun to set.
(C) They wanted to produce a variety of different factors.
(D) They wanted to make sure the weather was as cold as possible.