LETTUCE, 16tis, a genus (Lactuca) of hardy annual and perennial herbs of the order Composite. Less than 10 of the 100 recognized species, which are distributed mainly in the northern hemisphere, are in cultivation, and some botanists consider these to be merely forms of three or perhaps two species. They are characterized by opposite leaves of various forms, and white, yellow or blue flowers in heads which are arranged in small panicles. The only species used in America, the common garden lettuce (L. sativa), is an annual whose natural prototype is unknown, Thut is supposed to be L. scariola, an Asiatic species.
Lettuce is one of the oldest food-plants, having been used, it is said, by Persian royalty more than 2,000 years ago. To-day it is un questionably the most widely used of all our materials for salads, and has developed an in numerable host of varieties of great diversity of form. They are somewhat roughly divided into two general groups: heading, in which the leaves form a cabbage-like head; and cutting, in which the leaves are more loosely arranged. In each of these groups are forcing and out door varieties. A third group, the cos or romaine varieties, which may be considered a subdivision of the cabbage group, consists of long, narrow-headed kinds, whose outer leaves must be tied above the head to properly blanch the inner ones. They are specially valued as
summer lettuces, because of the ability to pro duce leaves of good flavor in spite of consider able heat.
Lettuce grows best in cool weather. It re quires a rich open soil and clean cultivation; plenty of sunlight in the cool seasons and par tial shade in the warm. The seed may be sowed in a hotbed, where the plants may be either alloived to develop, or from which they may be transplanted to the garden, 8 to 12 inches be ing allowed between them. Immense quantities of lettuce are forced in greenhouses during the winter, and from the trucking regions of the South, where the plants are grown with only sun heat in canvas-covered beds, the markets are supplied during the winter with thousands of carloads.
When grown under glass lettuce is some times attacked by so-called plant diseases leaf-spot, rust, mildew and drop or rot. These may be very largely if not wholly controlled by good management, especially with respect to ventilation, the temperature being kept lower than that which is favorable to the growth of the fungi. Sterilizing the soil with live steam for an hour or more is practised and believed by many large growers to destroy the spores. The operation is performed a day or so before planting.