Ecology Pollination and dispersal of ASTERACEAE MEMBERS

Ecology, Pollination and dispersal of ASTERACEAE MEMBERS:

Asteraceae are especially common in open and dry environments. Most Asteraceae are pollinated by insects, but anemophyly is also present (e.g. Artemisia).
Seeds are very often dispersed by the wind (anemochory) by mean of a hairy pappus. Another common variation is (epizoochory), in which the dispersal unit, a seed (e.g. Bidens)provided with hooks ( Xanthium), spines or some equivalent structure, sticks to the fur or plumage of an animal (or even to clothes) just to fall off later far from its mother plant.
The Asteraceae are one of the most widely spread families of the flowering plants. Their success like in their adaption to cross pollination by a large variety of insects. Small flowers are rendered conspicuous by aggregation into heads. The heads become further attractive by the development of the ray florets. A single visit of the insect may pollinate large number of flowers. The nectar is secreted by a ring shaped disc around the base of the style and collects in corolla tube which is accessible to a wide variety insects. In the absence of cross-pollination may occur in several genera in late stages of flowering. The stigma lobes come in contact with the style and brush the pollens which are retained by the stylar hairs.

Those fruits which have a pappus of plumes on hairs are adapted for wind dispersal. Hooked bristles on the pappus of Bidens and the development of hooked spines on the fruiting receptacles in Xanthium favour their distribution by birds and animals to the feather or fur which they cling.
Evolution: Diversification of Asteraceae may have been within 42-36 million years, the stem group perhaps being up to 49 million years old.
It is still unknown whether the precise cause of their great success was the development of the calathid, their ability to store energy as fructans (mainly inulin), which is an advantage in relatively dry zones, or some combination of these and possibly other factors.