Lactuca serriola.
It has 40 synonyms including Lactuca sativa subsp. serriola and L. scariola.
Native to Europe it is naturalised, and a common invasive weed in south-eastern Queensland.
The annual or biennial herbs reproduce only from seeds.
They mainly germinate in winter forming a short-lived basal rosette.
Plants starting in Spring usually have no basal rosette.
They have a deep taproot up to 80 cm long with fibrous roots.
The main growth period is Spring with plants reaching up to 2.5 m high.
With competition from other plants they only produce a single erect stem.
New plants may appear through Spring and into Summer.
The whitish-green stem may be smooth all over or have spines on the lower part.
Damaged parts exude a milky sap.
Leaves are alternate and have no petiole with the bases clasping the stem.
Leaves in the short-lived basal rosette are elliptic, ovate, obovate or oblong.
There are short spines on the slightly wavy edge.
Stem leaves change from the bottom up.
All have a base with 2 pointed lobes or auricles up to 2 cm long.
The lobes point down making them sagittate.
A rapidly growing stem may have some undivided leaves at the base.
The lower stem leaves are typically 10 to 15 cm long but occasionally up to 20 cm.
They are deeply pinnately dissected with 1 to 5 pairs of lateral lobes with tips that curve downwards (runcinate).
Leaves may twist at the base making the blade vertical.
The edges have small teeth and there are spines on the lower midrib and veins.
Leaves higher up the stem get smaller, less lobed and they occasionally have no spines.
The topmost leaves gradually change into small inflorescence bracts.
(L. serriola form integrifolia has stem leaves with no lobes).
Flowers are produced at the top of the stem in Spring to Summer.
Older stems that are flowering may have dense spines near the base.
Inflorescences are a large loose panicle with alternating lateral branches up to 20 cm or more in length.
At the base of each side branch is a much reduced leaf.
The lower branches have up to around 6 erect side branches, the upper branches have 1 or none.
At the end of each branch is a ligulate head of 25 to 30 small flowers (florets).
At the base of the head is an involucre with whorls of bracts (phyllaries).
The around 7 mm long involucre increases to 15 or 20 mm as the fruit develop.
The ends of the green phyllaries become purplish and they have a tuft of small hairs.
Florets have 5 fused pale yellow petals almost 1 cm long.
The tubular base, around 4 mm long has tiny hairs at the top.
The ligule of the 5 fused lobes is around 6 mm long with up to 5 small lobes.
The 5 free filaments insert onto the top of the corolla tube.
The longer anthers are fused into a tube around the style and bi-lobed stigma.
The anthers have long lower lobes and an apical appendage.
Florets are almost always self-pollinated.
The inferior ovary of each floret may develop into a cypsela with 1 seed.
Fruit are commonly and loosely called achenes which come from a superior ovary.
The single obovate fruit, around 4 mm long and 1 mm wide is flattened.
It is yellowish brown or grey with 5 slightly raised ribs and tiny spines on the tip and sides.
The broad end, with the seed, tapers to a paler narrower beak.
Attached to the beak is the pappus of 5 mm long hairs with minute barbs.
One plant can produce up to 100,000 seeds.
The cultivated species, Lactuca sativa and its many familiar varieties of lettuce are thought to have been derived from L. serriola.
J.F.
