How is lettuce digested in the digestive system?

Undigestable Fiber: Lettuce, and other green leafy vegetables are first mechanically digested via chewing, which breaks open the plant cell walls. The cellular contents are then absorbed by the intestinal linings. However, the cell walls themselves are not digestible (although the bacteria in the intestines can break them down somewhat). This is what constitutes fiber in the diet and forms the bulk in the stool.