We didn't harvest all of our lettuce earlier this summer. Some of it was left to grow.
It's interesting to see the entire life cycle of a vegetable from start to finish.
The lettuce gets bitter (you won't want to eat it after this), it sprouts (grows very tall), then flowers, and lastly the flowers produce seed. Technically, since this is an heirloom vegetable you could save these seeds to plant next year. However, without going into great detail, the resulting lettuce may not be identical to the parent due to cross-pollination. But if you are simply trying to save some money on next year's seeds and aren't really concerned that next year's lettuce is exactly the same variety, put those seeds in an envelope and store in a cool, dry place until next spring!
Here is what lettuce looks like when it goes to seed. You can harvest the seed anytime after the flowers die back and resemble a dandelion (below). (The heirloom lettuce varieties shown are freckles romaine, cimarron romaine, and Amish green deer tongue.)