Helianthus.
Family Asteraceae > Subfamily Asteroideae > Tribe Heliantheae > Subtribe Helianthinae.
Plants of the World Online (Kew) recognises 55 species while others around 50 to 70.
Almost all are native to North and Central America.
There are 5 naturalised species in Australia – H. argophyllus, H. annuus, H. ciliaris, H. debilis and H. tuberosus.
Some are annual but most are perennial with rhizomes (underground stems) with tubers.
The usually branched erect or creeping stems, up to more than 3 m high may have simple hairs.
Leaves can be concentrated at the base of the plant but are mostly spread along the stems.
The lower leaves are usually opposite and the upper ones alternate.
Leaves can be small or up to around 20 cm long and with or without a petiole.
The simple ovate, heart-shaped, linear or lanceolate leaves have a smooth edge or teeth.
There are usually 3 main veins and there may be no hairs or short stiff or soft ones on one or both surfaces.
Inflorescences, almost always solitary and terminal are a head or capitulum on a stalk or peduncle.
The involucre, at the base of the head has 3 (2 to 4) whorls of bracts or phyllaries.
The usually green bracts, equal or unequal in size can be ovate to lanceolate.
They can have a blunt or pointed tip and smooth or with hairs on the edges.
Some heads are only 1.5 cm across but most are large.
Each head, often called the ‘flower’ consists of many small tightly packed flowers or florets.
The few large outer or ray florets that look like the ‘petals’ surround the many disk florets.
The ray florets are typically yellow and the disk florets maroon or yellow.
Most species have radiate heads with both types of florets.
The florets are on a flat or hemispherical receptacle which is an upward extension of the peduncle.
The receptacle may have small deciduous scales or paleae known as chaff between the florets.
Palea have a pointed tip, 2 shorter side lobes and may have teeth on the tip and hairs.
The bilaterally symmetric ray florets, in 1 whorl are fertile or sterile females.
Their calyx consists of scales or barbed or feathery bristles collectively known as the pappus.
The 5 petals are fused into a corolla tube with a flat strap-like yellow (rarely orange) ligule.
The ligule, up to around 6 cm long usually has 3 lobes on the tip.
The radially symmetric disk florets are spirally arranged on the receptacle.
Bisexual and fertile they open from the edge inwards.
The calyx (pappus) consists or 2 or more scales.
The corolla tube has 5 equal brownish, yellowish or purplish lobes on the rim.
The 5 stamen filaments are free but the anthers are fused into a tube.
The basifixed anthers open inwards through longitudinal slits.
The anthers have a narrow triangular appendage at the top.
Florets have an inferior ovary of 2 carpels with one locule and one ovule.
The style collects the pollen as it passes through the anther tube then branches into 2 arms above the corolla tube.
The flattened branch ends have a linear stigma down the inner side and stiff hairs on the outer.
Each fertile flower produces a slightly flattened oblong to obovate, smooth or hairy seed.
Strictly speaking this is a cypsela (single seed from an inferior ovary) but commonly called an achene (from a superior ovary).
They may have a pappus of 2 or more usually deciduous scales.
J.F.
