Professor Prusinkiewicz is on leave in Australia，but he sent us an e­mail suggesting a simple way to understand the process.Think of a major road crowded with traffic.So many cars are on the main road that the cars on the side roads cannot enter.

　　The stem is like the crowded main road.The new research shows that the buds on the side cannot export their auxin into the main stem because it is too full.But if that main shoot is pruned(修剪)，other buds below it can start exporting.They are no longer prevented from growing.

　　Ottoline Leyser from the University of York says that after a plant is pruned，all the inhibited(被抑制的) shoot tips compete with each other to grow.In doing this，the branches influence each other's growth.Nearby shoot tips are more likely to affect each other than those that are far apart from each other.

　　Professor Leyser says the strongest branches grow best，wherever they may be on the plant.The study found that the main shoot grows the best of all not because of its position at the top of the plant，but mostly because it got there first.

　　【语篇解读】 本文讲述了关于降低植物顶端优势，修剪树木主干的新的理解。

　　5．The underlined part “two continents” in Paragraph 2 refers to “________”．

　　A．Asia and Africa

　　B．Africa and Europe

　　C．North America and Europe D．Australia and Europe

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