Family Compositae (Asteraceae)

Family Compositae (also called Asteraceae, and among the very largest flowering-plant families by number of species) consists mostly of herbs, distributed almost worldwide, and is defined above all by a single unmistakable inflorescence type: the capitulum, or flower head.

Vegetative characters. Members typically have alternate leaves (occasionally opposite), commonly showing reticulate venation, though some have parallel-veined leaves.

The capitulum. Rather than bearing individual, separated flowers, Compositae crowds together a large number of small, sessile florets on a single common receptacle, which may be flat or slightly convex; the entire structure is surrounded and protected by a whorl of green, leaf-like bracts called the involucre. Packed this tightly together, a capitulum can easily be mistaken at a glance for a single large flower, but it is in fact an entire inflorescence made up of many individual florets -- exactly the kind of highly specialised elaboration of the basic racemose inflorescence plan (an indeterminate axis bearing many flowers) introduced earlier in this chapter. Two distinct kinds of florets typically make up a capitulum. Ray florets are positioned around the margin of the head; they are zygomorphic, with their corolla drawn out into a single flattened, strap-shaped structure called a ligule, and they are usually either female or entirely sterile, their chief role being to make the head conspicuous to pollinators. Disc florets occupy the centre of the head; they are actinomorphic and bisexual, and it is these florets that show the family's characteristic floral structure most clearly.

Floral characters of the disc floret. The calyx is highly modified: rather than forming ordinary sepals, it develops into a ring of fine hair-like or scaly structures called the pappus, which persists into the fruiting stage. The corolla has five petals fused into a narrow tube (gamopetalous). The androecium has five stamens whose filaments remain free from one another, but whose anthers fuse together into a tube surrounding the style -- this specific condition, fusion of the anthers rather than the filaments, is called syngenesious, and is distinct from the various 'adelphous' conditions (which all describe filament fusion) seen in other families in this chapter. The gynoecium is bicarpellary and syncarpous, but with an ovary that is inferior -- positioned below the other three whorls rather than above them, unlike the superior ovaries of Malvaceae, Solanaceae and Brassicaceae -- unilocular, and containing just a single ovule attached by basal placentation. …