Have you ever wondered why stems(茎)grow upwards and roots downwards? Why do plants always seem to turn towards the light and climbing plants run up rather than down? The answer is simple: auxin(生长素), a chemical substance that controls growth in plants.
Auxin gives away its secrets
Auxin is a plant hormone(激素). Darwin was already interested in it in the 19th century. Only in recent years, however, has the hormone started to give away its secrets, thanks to intensive molecular research. Auxin is produced in the young, growing parts 
of plants and then transported throughout the plant—to a lowlying stem for example. The stem needs to straighten out as soon as possible to be able to absorb the topside, resulting in the underside growing faster and the stem straightening out. For the same reason, plants in front of windows will always turn to the light. This active regulation of auxin transport allows plants to take ideal advantage of local and changing conditions.
A new means of transport for auxin
The transport of auxin through the plant plays a vital role. And, from all appearances, it is not a simple matter. The researchers identified an important new link and means of transport for auxin: PILS proteins (蛋白质). PILS proteins are vital for auxindependent plant growth and adjust the intracellular(细胞内的)storage of the hormone. It is exactly this compartmentalizing (分类) of auxin that seems functionally important for the various developmental processes.
Growing crops more efficiently: the right amount of auxin in the right place
Higher auxin levels at the right moment and in the right place result in better growth and greater harvest. Better adjustment of auxin levels would make plants grow more efficiently. The researchers hope to contribute to the development of more efficient growing processes by continuing to sort out auxin transport processes.
66. Which of the following is TRUE of auxin according to the text?
A. It helps the plant to bend at the right places while growing.
B．It prevents the roots of plants from growing faster.
C．It is no use when the surroundings have changed.
D．It helps the underside of plants grow faster than the topside.
67. What would make plants grow more efficiently?
A. Straightening out the stem.
B．The right amount of auxin in the right place.
C．Allowing plants to turn to the light.
D．Enough PILS proteins.
68. From Paragraph 3 we know that ________.
A. the researchers are not so sure if the transport for auxin is a new one
B．the levels of auxin remain the same during the growing processes of plants
C．PILS proteins delay the growth of plants
D．PILS proteins play an important part in the transport of auxin
69．According to Paragraph 2, the researchers got their results mainly by ________.
A. referring to Darwin's theory
B．observing plants straightening
C．doing intensive molecular research
D．watching plants in different places
70．Which of the following can be the best title for this passage?
A. Why Plants Grow Towards the Light?
B．Darwin's Dream Has Come True
C．How Did Scientists Find Out the Secret?
D．A New Way of the Growing of Plants