An ovule is a tiny structure in a plant that develops into a seed after fertilization. Inside a flower, ovules sit protected within the ovary, waiting for pollen to arrive. When pollen reaches the ovule and fertilization occurs, that little ovule begins transforming into a seed containing a baby plant and its food supply.

Think of an ovule as a plant's egg. Just as a chicken's egg can develop into a chick, an ovule can develop into a seed that will eventually grow into a new plant. If you cut open an apple, those brown seeds you see were once ovules. The same goes for the seeds inside a tomato, the pit inside a peach, or the kernels on an ear of corn.

Different plants protect their ovules in different ways. Flowering plants keep them safely tucked inside an ovary (which often becomes the fruit), while pine cones hold their ovules openly on their scales. This is why pine trees are called “gymnosperms,” meaning naked seed, while apple trees are “angiosperms,” meaning enclosed seed.