roots are a vehicle for absorbing water from soil into the plant & if something is a vehicle for a process then that something performs that process & absorbing water from soil is a kind of process ->  roots absorb water from soil  roots absorb water from soil & absorbing is a kind of function ->  a function of roots is to absorb water  a function of roots is to absorb water & absorbing something is a kind of method for taking in something -> a function of roots is to take in water