Stems grow against gravity. This is called geotropism, and it's a plant's way of reaching for sunlight.

* Positive geotropism: Roots grow *downward* towards gravity, which helps them anchor the plant and access water and nutrients.

* Negative geotropism: Stems grow *upward* against gravity, which allows them to maximize exposure to sunlight for photosynthesis.

However, there are exceptions to this rule. For example, some climbing plants have stems that grow horizontally or even downwards, using other structures for support.