A carbohydrate that occurs as discrete, partially crystalline granules in the seeds, roots (tubers), stems (pith), leaves, fruits, and pollen grains of higher plants. Starch functions as the main storage or reserve form of carbohydrate; it is second in abundance only to cellulose, a major structural component of plants. Cereal grains, tuber and root crops, and legumes (seeds) have long been used as major sources of carbohydrate in human diets.