Prickly lettuce (Lactuca serriola L. or Lactuca sativa L. subsp. serriola (L.) Galasso, Banfi, Bartolucci & Ardenghi) is edible in small quantities: few leaves (when young) are usually mixed with other wild or cultivated leaves, making a tasteful and mellow salad. The slightly bitter taste is quite pleasant.
When the stalks and leaves are cut, the plants secretes a white latex that thickens and turns slightly brown upon drying. The fresh latex seems to be sligthly toxic, but the dried one (called lactucarium) has medicinal and slightly psichoactive properties. During the XIX century, lactucarium was used as an opium substitute (called lactuca opium in Latin).
The plants exposed in full sun move the leaves in such a way to align their margins along the North-South direction and to orientate the leaf blades toward South-East, in order to avoid excessive sunlight. This way the leaves receive sunlight only on the margin at midday and on the blades in the morning and the evening. For this reason, this herb is also called “compass plant”.