Tithonia.
Family Asteraceae > Subfamily Asteroideae > Tribe Heliantheae > Subtribe Helianthinae with 19 genera including:
Helianthus which have solid peduncles (flower stalks) that are not swollen at the top and a pappus of 0 to 5 deciduous scale-like awns (bristles) and
Tithonia with swollen, hollow peduncles and a pappus of persistent fused scales and 2 awns.
The 12 or 14 species of Tithonia are mostly from Mexico and Central America.
Two species are widely cultivated and naturalised in Australia.
Tithonia rotundifolia are annual herbs with flower heads 4 (9) cm across and orange ray ligules 1 to 3 cm long.
T. diversifolia are perennials with heads 10 to 15 cm across and yellow ray florets 4 to 7 cm long.
Apart from one tree Tithonia are erect annual or perennial herbs or shrubs.
The leaves are alternate but the lower ones may be opposite.
Leaves, with or without a petiole are lobed from the base (palmate) or along a midrib (pinnate) and have 3 main veins.
Terminal inflorescences are on a long stalk or peduncle that is wider and hollow at the top.
The flower heads have both ray and disk florets.
The involucre under the heads has up to 5 whorls of bracts.
The receptacle holding the florets has scales or paleae.
Sterile ray florets have a ligule with 2 or 3 teeth.
Bisexual disk florets have 5 small lobes on a tubular base and fused anthers with a free apical appendage.
The style on the inferior ovary has 2 stigmatic branches.
The cypselas (achenes) have a pappus of scales and 2 longer awns or bristles.
J.F.
