Sweet peas, cucumbers, grapes ---- what do you think they have in common? None of these vegetables grow on trees. They all don't have strong roots, big trunks or long branches, which makes it harder for them to get sunlight. But don't worry. They have developed their own unique way of growing taller climbing! Take sweet peas as an example. The plant grows as a straight stem until it finds something to hold onto, either a nearby tree or a stick. That is when the stem makes small coil  that helps the plant hold the stick tightly. How these plants are able to do this has been a mystery to scientists for years. Even Charles Darwin was interested in this question. But it is not until now that a Harvard research team has finally found the answer. The study was published in the Journal Science earlier this month. The key, as it turns out, is inside a kind of thread-like cell called "g-fiber cell", which has the ability to shrink or stretch. Each stem of the pea plant is made of several layers of g-fibers. Once the plant finds something to hold onto, cells on side of the stem start to shrink, forcing the whole thing to bend and coil, which is a bit like how our muscles get our bodies to move. What's more, a pea's coil is not exactly like that of a spring. If you look closely, you may find that a spring coils all in the same direction, but a plant's coil is in two opposite directions, like a telephone line. This means that the stem can stay soft when pulled gently but become strong enough when pulled harder. "You want the plant to have a little bit of flexibility  so that if the wind blows or an animal disturb it, it doesn't break," explained Sharon Gerbode, author of the study. "This structure allows the plant to easily accommodate small movement, but then if something really serious happens it can become strong and protect itself." According to the article, which one is true about "a sweet pea's coil"?
to be like that of a spring to bend all in the same direction to make the stem hard break to make the stem get soft when pulled hard