a plant stem is the vehicle for transporting water and food from roots to the rest of the plant & if something is a vehicle for a process then that something performs that process ->  a plant stem transports water and food from the roots to the rest of the plant  a plant stem transports water and food from the roots to the rest of the plant & transporting materials through a plant is a kind of function -> a function of a plant stem is to transport water and food from the roots to the rest of the plant