- Roots are a vehicle for absorbing water and nutrients from soil into the plant
- If something is a vehicle for a process then that something performs that process
- Therefore, roots absorb water and nutrients from the soil

- Roots absorb water and nutrients from the soil
- Absorbing is a kind of function
- Therefore, the function of roots is to absorb water and nutrients

- The function of roots is to absorb water and nutrients
- Some minerals are nutrients
- Therefore, the function of roots is to absorb water and minerals

So, the function of roots is to absorb water and minerals
A function of a plant's roots is to
- Roots are a vehicle for absorbing water and nutrients from soil into the plant 
- Some minerals are nutrients 
- If something is a vehicle for a process then that something performs that process 
- Absorbing is a kind of function