A lettuce plant has a short stem initially, but when it blooms, the stem lengthens and branches, and it produces many flower heads that look like those of dandelions, but smaller. This is called "bolting". Some lettuces (especially iceberg) have been specifically bred to remove the bitterness from their leaves. These lettuces are largely water with very little nutrient value. The more bitter lettuces and the ones with pigmented leaves contain anti-oxidants.