A type of seed sometimes dropped by plants grown on a Crop Block. It can be used in the Cropnalyzer to give information on the plant it will grow. These are also used to make hybrid plants. If you place three crop blocks in a row and grow two plants on the outside with seed bags, the hybrid will be grown in the middle crop block.
You get seedbags from left clicking fully grown plants grown using the crop sticks. (Not from placing a wheat in the center of a crafting table.)