The key ingredient of most salads, lettuce has been cultivated for more than 4000 years along the Nile in Egypt. Christopher Columbus is responsible for introducing the vegetable to the Americas, where it has been grown since 1494. Several different varieties of lettuce exist, including head lettuce, loose-leaf lettuce, and romaine. The well-known iceberg type, which is an example of head lettuce and is common in supermarkets across the United States, was reportedly developed in 1894. More crisp and less perishable than other types of lettuce, iceberg helped make the vegetable a year-round, rather than a seasonal, presence at the dinner table. On the downside, however, iceberg lettuce is mostly water with some fiber, offering little nutritional value when compared with romaine and most other types of lettuce.