This is the VOA Special English Agriculture Report.

We talked last week about when and how to prune plants. 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 nineteen thirties that the actively growing tip of a plant releases a hormone called auxin. This hormone flows down the main stem. Scientists say the auxin has an indirect effect on buds on the side of the stem to prevent branching.

These 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 sabbatical 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, other buds below it can start exporting. They are no longer inhibited from growing.

Ottoline Leyser from the University of York says that after a plant is pruned, all the inhibited 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.

And that's the VOA Special English Agriculture Report, written by Jerilyn Watson. Transcripts and MP3s of our reports, including last week's advice about pruning, can be found at voaspecialenglish.com. You can also post comments. We invite you to share your own experiences -- good or bad -- with pruning plants and trees. I'm Steve Ember.