Starch provides long-term energy storage for plants. The energy for plants lies in the sugar molecule glucose. Glucose that is not used immediately can be stored in the roots and seeds as a branching-coiled molecule called starch.
Starch is a polysaccharide that actually consists of two types of molecules: amylopectin and amylose. Amylopectin consists of branching-glucose molecules linked together whereas amylose is more linear. Polysaccharides are good energy storage molecules because they can be broken down quite easily and are more compact than glucose molecules.
A single starch molecule, depending on the kind of starch it is, can contain 500 or even a few hundred thousand glucose molecules. When glucose is needed for energy, the plant uses enzymes to break down starch into its constituent glucose molecules.