The size of the stem increases in the width due to the meristem.

Intercalary

Explanation:

There are three types of meristems : apical, lateral and intercalary.

These are found at different parts of the plant, but their ultimate job is to multiply and so increase the length of the plant.

The length of the stem is determined by the apical meristem, which is found at the tip of the plant.Of this meristem múltiples, the plant grows taller.

The root of the meristem is determined by the lateral meristem, which is found at the bottom of the plant, near the roots.

The thickness of the plant is determined by the intercalary meristem, which is found at the centre of the plant, and when they multiple, the plant increases in width.

Hope this helps.

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