How plants keep other plants away...

Allelopathy is a chemical process. There are several types of chemical alleopathy. In one kind, the plant that is protecting its space releases something called growth-compounds, from its roots into the ground. New plants trying to grow around the allelopathic plant absorb those chemicals in the soil and are unable to live. A second type of allelopathy releases chemicals that hurt the way growing plants carry out photosynthesis. An allelopathic plant may also release chemicals that change the amount of chlorophyll a plant has in it. When a plant's chlorophyll levels are changed, it cannot make the food it needs, and the plant dies.

There are several ways a protecting plant can release their protective chemicals.

Volatilization
Protector trees release a chemical in the form of a gas through small openings in their leaves. If other plants absorb the toxic chemical, it will die.
Leaching
All plants lose leaves. Some plants release their protective chemicals in the leaves they drop. When the leaves fall to the ground they decompose. When they decompose, the leaves give off chemicals that protect the plant.
Exudation
Some plants release their defensive chemicals through their roots. These chemicals are given off into the soil. The chemicals are absorbed by the roots of plants close to the allelopathic one.

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