Here is some vegetable seeding info I know:
1) Broccoli- Seeds form after the broccoli flowers. Most of the time you don't see broccoli flowers because it's picked green. Even though sometimes the broccoli flowers after it is cut from the stock it will not produce seeds once it has been cut from the stock.
2) Carrots- The stem of the carrots will produce flowers, which will produce seeds once the flowers are done blooming. The seeds are very small.
3) Potato, you can start a potato with the "eye" of a potato. That's the root that grows out of the potato if it is left in your cupboard for too long.
4) Celery- The leafy part at the top will flower and seed. When the seeds are showing, cut the celery, put it in something to dry (like a paper bag). Once the seeds dry they'll fall off with a shake.
Hope that helps.
at 3:03 AM on Jan. 30, 2011