Here's how it works:

* Water is essential for plant growth: Plants need water to transport nutrients, maintain turgor pressure (the stiffness of their cells), and carry out photosynthesis.

* Roots sense moisture gradients: Root tips contain specialized cells that can detect differences in water concentration in the soil.

* Hormonal response: When roots encounter a drier area, they produce the plant hormone auxin on the side facing the moisture source.

* Growth response: Auxin stimulates cell elongation on the side facing the water source, causing the root to bend and grow towards the moisture.

So, the answer is: Plant roots grow towards the source of water because of hydrotropism, a growth response triggered by the detection of moisture gradients and the production of the hormone auxin.