Think of the structure of a flower as you read through the answer and it will help you visualize the process of seed formation. A pollen grain produced from a stamen (male part of flower) will land on the top of a carpel (the female part of the flower that sticks up in the middle). A pollen tube will grow from the grain of pollen down into the carpel where it will come in contact with an ovule. Two sperm that were in the grain of pollen travel down the pollen tube into the ovule. Inside the ovule there are two different types of cells, the egg and the embryo sac. One sperm combines with the egg to make the part of the ovule that will be the new plant, also called the embryo. The other sperm combines with the embryo sac to make endosperm which will be a food source for the plant when it begins to grow.
Before the ovule (which now contains the embryo and embryo sac) becomes a "seed" the endosperm grows and becomes solid, and the embryo divides into a top half that will form the shoots of the plant, and the bottom half that will become roots. It also dehydrates until it only 5 to 15% of its total weight is water. The embryo inside the seed will stop growing until the seed is planted, and the fleshy material surrounding the ovule/seed (the ovary) will develop into a fruit that will help disperse the one or many seeds inside of it.
This basic explanation is true for flowers as well for plants without traditional flowers (lacking petals), such as corn. With corn, each kernel on the ear of corn is an individual seed, and the ear is one large carpel. The stamen of the corn that fertilize the carpel are the tassels at the tip of the plant. This structure is slightly modified from that of a traditional flower, but the mechanics of seed formation are the same.
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