The romaine lettuce Unlike most other lettuces tolerates heat very well
I didn't know that. I like it the best. The darker green type of lettuce
Lettuce
Lettuce ("Lactuca sativa") is an annual plant of the daisy family, Asteraceae. It is most often grown as a leaf vegetable, but sometimes for its stem and seeds. Lettuce is most often used for salads, although it is also seen in other kinds of food, such as soups, sandwiches and wraps; it can also be grilled. One variety, the " (), or asparagus lettuce (celtuce), is grown for its stems, which are eaten either raw or cooked.
Romaine lettuce
Romaine or cos lettuce ("Lactuca sativa" L. var. "longifolia") is a variety of lettuce that grows in a tall head of sturdy dark green leaves with firm ribs down their centers. Unlike most lettuces, it is tolerant of heat. In North America, romaine is sold as whole heads or as “hearts” that have had the outer leaves removed and are often packaged together. 
Commercially sold romaine lettuce has historically been the subject of product warnings by both U.S. and Canadian health authorities