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. As of November 2018[update], commercially sold romaine lettuce is the subject of warnings by U. S. and Canadian health authorities that it should not be consumed in any form, at home or in restaurants, by humans due to contamination with E. coli bacteria.