Lettuce (Lactuca sativa) is generally divided into four categories, based on shape and growth patterns. All are excellent in salads and can be easily substituted for one another in most recipes.
COS, OR ROMAINE Lactuca sativa longifolia). Upright and cylindrical with thick, crisp juicy leaves. Types include Valmaine, Paris White, Jericho, Rouge d'Hiver (red) and Diamond Gem (a miniature). Higher in vitamins and minerals than crisphead or butterhead lettuces.
LOOSE-LEAF (Lactuca s. crispa). Abundant leaves that can be harvested singly; tender and flavorful. Cultivated in many colors, most commonly green and red; reds provide deeper flavor. Popular varieties include Oak Leaf, Pirat, Salad Bowl and Biondo Lise.
CRISPHEAD, OR ICEBERG (Lactuca s. capitata). Tight leaves, very mild flavor. Excellent for added texture and when a sturdier leaf is desired. Good varieties include Red Grenoble, Green Ice and Great Lakes.
BUTTERHEAD Lactuca s. capitata). Smooth, buttery, very tender leaves that grow in a rose-shaped head. Bruises easily. Includes Boston, Bibb, Four Seasons (red and green outer leaves) and Tom Thumb (a miniature).
When buying lettuce, look for robust color. Deep-green indicates health and freshness; yellow or brown spots may signal age and reduced nutrients. All greens perish quickly; if possible, use lettuce the day it's purchased. Wash leaves gently and thoroughly and dry with a salad spinner or by placing rinsed leaves in a mesh bag and twirling it outside.
Sources: The Greens Book (Interweave Press, 1995) by Susan Belsinger and Carolyn Dille; Lettuce in Your Kitchen (William Morrow and Company, 1996) by Chris Schlesinger and John Willoughby.