ASTERACEAE - the sunflower family

ASTERACEAE - the sunflower family

Systematic position

Class: Dicotyledonae

Sub-class: Gamopetalae

Series: Inferae

Order: Asterales

Family: Asteraceae

General characters

Distribution

Asteraceae is the largest family of flowering plants comprising about 900 genera and more than 20,000 species. They are distributed throughout the world. In India, about 138 genera and about 708 species are reported.

Habit

Mostly annual or perennial herbs (eg. Eupatorium odoratum) or shrubs

(eg. Senecio) or trees (eg. Vernonia arborea). Root and stem commonly contain oil ducts. Many species have colourless latex.

Root

Normally a branched tap root system. Root tubers are found in Dahlia coccinea.

Stem

Aerial, erect or prostrate or decumbent, tuberous (eg. Helianthus tuberosus) or sucker eg. Chrysanthemum indicum (Akrakaram) or runner (eg. Launaea pinnatifida).

Leaf

Simple, entire or pinnately or palmately lobed or compound, alternate (eg. Vernonia arborea) or opposite (eg. Tridax procumbens) or whorled (eg. Eupatorium odoratum) or radical (eg. Launaea pinnatifida), exstipulate showing reticulate venation, hairy (Tridax procumbens) and spinous (eg. Carthamus tinctorius).

Inflorescence

This family is characterized by the presence of head or capitulum type of inflorescence. The number of florets in a head is variable from a few to several hundreds, but in Echinops, the head is reduced to a single flower.

The heads are of two types, heterogamous and homogamous. There are two types of florets in heterogamous heads eg. Helianthus annuus and Tridax procumbens (Vettukkaya plant). The central florets are tubular, bisexual and also known as disc florets. The marginal florets are ligulate, pistillate and also known as ray florets. In homogamous heads, all the florets are tubular as in Vernonia arborea and Ageratum mexicanum.

All the florets are ligulate as in Launaea pinnatifida.

Flower

Very small in size hence referred to as florets, sessile, bracteolate (eg. Helianthus annuus) or ebracteolate (eg. Tridax procumbens).

Pentamerous and epigynous. The disc florets are generally complete, bisexual and actinomorphic, whereas the ray florets are incomplete, pistillate or neutral and zygomorphic.

Calyx

In many species calyx is reduced to numerous hairy appendages called pappus, arranged on top of the ovary. The persistent pappus acts like a parachute and helps in the dispersal of fruit.

Corolla

Petals 5, gamopetalous showing valvate aestivation, regular and tubular in disc florets and irregular and ligulate or bilabiate in ray florets.

Androecium

Stamens 5, epipetalous and alternate the petals. The stamens are syngenesious i.e. only anthers are fused into a tube around the style and filaments are free. Anthers are dithecous, basifixed and dehiscing longitudinally.

Gynoecium

Ovary inferior, bicarpellary and syncarpous. Ovary unilocular with a single ovule on basal placentation. Style simple with bifid stigma.

Fruit

Cypsela, single seeded, dry indehiscent fruit developing from an inferior ovary.

Seed

Non-endospermous.