Lettuce is a fairly hardy, cool-weather vegetable that thrives when the average daily temperature is between 60 and 70Â°F. It should be planted in early spring or late summer. At high temperatures, growth is stunted, the leaves may be bitter and the seedstalk forms and elongates rapidly. Some types and varieties of lettuce withstand heat better than others. There are five distinct types of lettuce: leaf (also called loose-leaf lettuce), Cos or romaine, crisphead, butterhead and stem (also called asparagus lettuce). Leaf lettuce, the most widely adapted type, produces crisp leaves loosely arranged on the stalk. Nearly every garden has at least a short row of leaf lettuce, making it the most widely planted salad vegetable. Cos or romaine forms an upright, elongated head and is an excellent addition to salads and sandwiches. The butterhead varieties are generally small, loose-heading types that have tender, soft leaves with a delicate sweet flavor. Stem lettuce forms an enlarged seedstalk that is used mainly in stewed, creamed and Chinese dishes. Crisphead varieties, the iceberg types common at supermarkets all over the country, are adapted to northern conditions and require the most care. In areas without long, cool seasons, they generally are grown from transplants, started early and moved to the garden as soon as the soil can be worked. They are extremely sensitive to heat and must mature before the first hot spell of summer to achieve high-quality heads. If an unseasonably early heat wave hits before they have matured, they almost certainly fail. In many locations, crisphead lettuce plants started in late summer to mature in the cooler weather of fall have a much better chance of success. The nutritional value of lettuce varies with the variety. Lettuce in general provides small amounts of dietary fiber, some carbohydrates, a little protein and a trace of fat. Its most important nutrients are vitamin A and potassium. The vitamin A comes from beta carotene, whose yellow-orange is hidden by green chlorophyll pigments. Beta carotene, of course, is converted to vitamin A in the human body. The darker green, the more beta carotene. According to the American Cancer Institute and the American Cancer Society, foods rich in vitamin A and C (antioxidants) offer protection against some forms of cancer. Along with other phytochemical, antioxidants reduce the risk of cancer of the respiratory system and intestinal tract. Lettuce, except iceberg, is also a moderately good source of vitamin C, calcium, iron and copper. The spine and ribs provide dietary fiber, while vitamins and minerals are concentrated in the delicate leaf portion.