Sentence:  This allows a weak plant to benefit from the strong roots of another, or a vigorous tree (such as an <WSD>apple</WSD>) to be kept small by growing on 'dwarfing rootstock'. 
Meaning of the word marked with <WSD> tags:  A tree of the genus "Malus", especially one cultivated for its edible fruit; the apple tree. (from 15th c.)