Lactuca serriola, also called prickly lettuce, milk thistle (not to be confused with Silybum marianum, also called milk thistle) compass plant, and scarole, is an annual or biennial plant in the dandelion tribe within the daisy family. It is slightly foetid, and is commonly considered a weed of orchards, roadsides and field crops. It is the closest wild relative of cultivated lettuce (Lactuca sativa L.).
Lactuca serriola is known as the compass plant because in the Sun the upper leaves twist round to hold their margins upright.
Lactuca serriola has nine dominant resistance genes to Bremia lactucae, or downy mildew.
Lactuca serriola has a spineless reddish stem, containing a milky latex, growing from 30 to 200 cm (12-80 inches). The leaves get progressively smaller as they reach its top. They are oblong or lanceolate, often pinnate and (especially for the lower leaves), waxy grey green. Fine spines are present along the veins and leaf edges. The undersides have whitish veins. They emit latex when cut. The flower heads are 11 to 13 mm (0.44-0.52 inches) wide, pale yellow, often tinged purple, with 12-20 ray flowers but no disc flowers. The bracts are also often tinged purple. It flowers from July until September. The achenes are grey, tipped with bristles. The pappus is white with equal length hairs.
Culinary and medicinal uses
Lactuca serriola can be eaten as a salad, although it has something of a bitter taste. Young leaves can be eaten raw or cooked. However, its presence in some ancient deposits has been linked more to its soporific properties which might suggest ritual use. The Ancient Greeks also believed its pungent juice to be a remedy against eye ulcers and Pythagoreans called the lettuce eunuch because it caused urination and relaxed sexual desire. The Navajo used the plant as a ceremonial emetic. In the island of Crete in Greece the leaves and the tender shoots of a variety called maroula (μαρούλα) or agriomaroulo (αγριομάρουλο) are eaten boiled. It is used by a growing number of Jews and the Samaritans as the Maror (bitter herb) on Pesach.
The Egyptian god Min is associated with this species of lettuce. Also, archaeobotanical evidence in Greek archaeological contexts is scanty, although uncarbonised seeds have been retrieved from a 7th-century BC deposit in a sanctuary of Hera on Samos. It is also described by Theophrastus. In mythology, Aphrodite is said to have laid Adonis in a lettuce bed, leading to the vegetable's association with food for the dead. 
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