Today we tell you about some new understanding of why cutting the main branch of a plant
or tree can lead to better development.
     The findings are from researchers on two_continents.Professor Prezemyslaw Prusinkiewicz
of the University of Calgary in Canada led the research with scientists from Britain and Sweden.
Their study appears in the Proceedings of the National Academy of Sciences.
     Researchers have known since the 1930s that the actively growing tip of a plant produces
a hormone(荷尔蒙)called auxin(生长素)．This hormone flows down the main stem.Scientists
say the auxin has an indirect effect on buds(芽)on the side of the stem to prevent branching.
T     hese buds themselves also produce auxin.The research suggests that to grow,they have
to be able to export the hormone into the main stem.But the flow from the stem tip prevents
them from doing this.The researchers wanted to find out how this blocking happens.
     Professor Prusinkiewicz is on leave in Australia,but he sent us an e－mail 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.

A．Researchers have recently found out that the growing tip of a plant produces hormones.
B．Auxin has a direct effect on buds to keep branches from growing.
C．Buds on the side of the stem can’t produce hormones.
D．In order to grow，the buds on the side of the stem export the hormones into the main stem.