The Mystery of Why Sunflowers Turn to Follow the Sun—Solved Scientists have answered a burning question central to sunflowers：Why do young flowers always face the sun over the course of a day？
   And also：Once sunflowers become full-grown，why do they stop tracking the sun and only face east？
   In a newly-published article，researchers claim that a young plant's sun-tracking （also called heliotropism）can be explained by circadian rhythms—the behavioural changes tied to an internal clock that humans also have，which generally follows a 24-hour cycle.
   "Growth rates on the east side were high during the day and very low at night，whereas growth rates on the west side were low during the day and higher at night，" the journal article says.
   The researchers tied plants up so they couldn't move or turned them away from the sun—and they found those flowers eventually had "fewer organic materials and less leaf area" than flowers that could move with the sun.
   Full-grown sunflowers respond differently to the sun.According to the article，"As overall growth slows down，the circadian clock ensures that the plant reacts more strongly to light early in the morning than in the afternoon or evening，so it gradually stops moving westward during the day."
   The researchers compared full-grown flowers facing east with those they turned to face west，and found that east-facing flowers attracted five times as many helpful pollinators.That's because the east-facing flowers heat up faster and bees like warm flowers.
   "Just like people，plants depend on the daily rhythms of day and night to function，" Anne Sylvester，director of the National Science Foundation's Plant Genome（基因組） Research Programme said in a press release. "Sunflowers，like solar panels，follow the sun from east to west.These researchers explore information in the sunflower genome to understand how and why sunflowers track the sun."
   UC Berkeley professor and co-author of the study，Benjamin Blackman，believes that the connection between circadian rhythms and growth could be applicable to other researches. "The more general point，that one of the circadian clock's adaptive functions is to regulate the timing and strength of growth responses to environmental signals，is one that I think will apply to a broad range of qualities and species，" he said.

（1）What did the researchers discover when they tied the sunflowers up？

C

C

A．They still tracked and faced towards the sun.
B．They grew faster during the daytime than they did at night.
C．Their growth rates were lower than those of flowers facing the sun.
D．The growth rates on the west side were higher than those on the east.
（2）Why do sunflowers move westward less when they are fully-grown？

D

D

A．Because they will not be affected by light anymore.
B．Because they would stop growing up if they faced west.
C．Because they are not active at all during afternoons or evenings.
D．Because they are more sensitive to light in the early morning from the east.
（3）What may the underlined word "pollinators" （Para.7）refer to？

A

A

A．Bees.
B．Plants.
C．Energy.
D．Researchers.
（4）What is the possible direction of future study，according to Benjamin Blackman？

D

D

A．Exploring information in the sunflower genome.
B．Comparing human's circadian rhythms with sunflowers'.
C．Comparing full-grown flowers facing east with those facing west.
D．Exploring how circadian rhythms affect the growth of other species.