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 has a slightly fetid odor 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 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.
Lactuca serriola can be affected by lettuce downy mildew, one of the most serious diseases of lettuce. To withstand this, L. serriola has shown resistance to the plant pathogen Bremia lactucae, the cause of the disease.This pathogen is able to undergo sexual reproduction, and once virulent strains have been produced, can undergo rapid asexual reproductive cycles. As a result, there are many strains, each of which vary in virulence. 
Resistance to Bremia lactucae in Lactuca serriola is characterized by Dm genes, or single dominant genes. Nine of the dominant genes that confer resistance are Dml, Dm, Dm3, Dm6, Dml4, Dml5, DmlO, Dm5/8, Dm10, Dm4, Dm7, Dm11, and Dm13. These genes are mapped in four linkage groups, so the genes within each group will be more likely to be inherited together. Lactuca serriola and B. lactucae have a gene-for-gene relationship, meaning that each resistance gene in the plant is associated with a specific gene in the pathogen, with avirulence being dominant to virulence. The possible combinations of these Dm genes can provide the plant with resistance to multiple strains of Bremia lactucae.
Testing for the presence of new resistance factors is conducted by screening samples of L. serriola with various isolates of B. lactucae. Samples of Lactuca serriola can be found around the world with little overlap in their genotypes, showing that there is genetic diversity between populations regarding the Dm genes of L. serriola. This genetic diversity is considered a resource for lettuce breeding because it provides a greater variety of genes to be used in response to new strains of B. lactucae. There is especially high diversity within the Mediterranean area and Southwest Asia, but L. serriola has established populations on all continents and has the most widespread distribution compared to other Lactuca species. Knowledge about genetic resources in Lactuca serriola is useful for lettuce breeding, allowing breeders to effectively respond to the pathogen.  This is especially applicable because L. serriola is the wild progenitor of cultivated lettuce (Lactuca sativa). In addition, these resources are necessary as new strains of the pathogen continually emerge.
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