This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
There is no trace of Wall Lettuce in ancient deposits. It is found in the North Temperate Zone in Europe and Western Asia. In Great Britain it is found in the Peninsula, Channel, Anglia provinces, except in Bedford, and Hunts in the Severn province; in S. Wales, except in Radnor; in N. Wales, except in Merioneth and Montgomery; in the Trent province; in the Mersey province, except in Mid Lanes; the Humber and Tyne provinces, except in Cheviotland; the Lakes province, except in the Isle of Man; in Scotland, in Ayr, E. and W. Perth, Easterness, Westerness, Argyle. In England it is rare, ascending to 1300 ft. in Yorks. It occurs in Wicklow and Louth in Ireland.
Wall Lettuce, as implied by the name, is a mural species which grows on walls and rock ledges. It is found in the crevices of perpendicular faces of damp rocks in woods, well sheltered by overhanging trees or vegetation, or on stony banks. It has an erect stem, with peculiarly delicate leaves, with lobes larger upwards, runcinate, with lobes each side of a common stalk, with a channelled leaf-stalk clasping the stem, with angles and fine teeth, the terminal lobe the largest, five-angled. The keel is smooth.
The flowerheads are pale-yellow, and very small, having only 5 florets growing in erect panicles, which are spreading. The beak of the fruit is shorter than the fruit.
The plant is about 2 ft. high. It is in bloom in July. Perennial it can be propagated by division, and is really worth growing in a garden.
The flowerheads are yellow, rather small and distant, and the plant grows in the shade, and is little likely to be visited very largely by insects. The florets are few and hermaphrodite. The stamens are hair-like and short, and the anthers, which are yellow, form a cylinder. The style, longer than the stamens, is thread-like, with a stigma with 2 lobes, which is turned back. Thus the structure is quite like that in most Composites, but self-pollination is more likely to occur.
The silky, simple, white pappus is light, and the achenes are dispersed by the wind.
Wall Lettuce is a rock plant growing on rock soil. The type of rock varies, but is largely sandy, and not usually calcareous. Thus it grows on Precambrian slate and on arenaceous marlstone of the Lactuca, Pliny, is from lac, milk, in allusion to the milky juice it secretes. Wall Lettuce is the only name by which it is known.