Romaine lettuce is high in water content and provides 2 grams of fiber per serving.  It is also rich in the B vitamins, vitamin A, C, and K, and is a good source of calcium and iron.  It provides other minerals such as copper, magnesium, manganese, phosphorus, potassium, selenium and zinc.  Store in open top plastic bag, wait to wash until just before using.   One suggestion is to wrap the bottom (root end) in a damp paper towel and store upright in a large plastic bag.