Lettuce root. The tap roots of young lettuce plants, Lactuca sativa. Root hairs are visible which form an intimate contact with the soil particles, and enormously increase the root's absorbing surface area. After seed germination of Lactuca sativa, the tap root develops quickly and grows to a depth of up to 1.5 metres. Lettuce is the most widely cultivated salad crop, growing best in well-drained and watered soils.