11th Class Biology Families Compositae (Asteraceae)

Compositae (Asteraceae)

Category : 11th Class

Systematic position

        Division      :         Angiospermae

        Class            :         Dicotyledonae

        Subclass     :         Gamopetalae

        Series          :         Inferae

        Order          :         Asterales            

        Family         :         Compositae (Asteraceae)

                                    (Largest family among the angiosperms)

Habit : Most of the plants are annual herbs (e.g., Chrysanthemum, Lactuca, Calendula, Helianthus, Tagetes). A few are shrubs (e.g., Artemisia, Pluchea lanceolata) or rarely trees (e.g., Vernonia arborea, Wilkesia, Leucomeris). Milkamia cordata is a twiner.

Root : Usually there is a tap root, but in Dahlia and Taraxacum officinale fasciculated roots are present.

Stem : Stem is usually herbaceous, erect, branched, solid, fibrous and sometimes with milky latex. In Jerusalem artichoke (Helianthus tuberosus) the stem is underground and tuberous. In Baccharis, it is winged like a leaf.

Leaves : Leaves are mostly alternate and occasionally opposite (e.g., Helianthus) or whorled (e.g., Eupatorium, Zinnia verticillata). They are exstipulate, petiolate, simple, pinnately or palmately lobed or compound (e.g., Dahlia, Cosmos). Venation is reticulate.

Inflorescence : Inflorescence is capitulum or head with an involucre of bracts at its base. The number of flowers in each inflorescence varies from 1000 (in large flowers of Helianthus) to 1 (in Echinops). Peduncle flat on which florets are attached.

Flower : Epigynous, usually pentamerous with reduction in certain whorls, hermaphrodite or unisexual complete or incomplete, tubular (actinomorphic) or ligulate (zygomorphic), bracteate or ebracteate.

(1) Ray florets : Towards periphery of head, sessile bracteate, pistillate or neutral, zygomorphic, ligulate, epigynous.

Calyx : Absent or hairy pappus or scaly, persistant.

Corolla : Petals 5, gamopetalous, ligulate, strap shaped.

Androecium : Absent.

Gynoecium : Bicarpellary, syncarpous, ovary inferior, unilocular, one ovule in each locule, basal placentation, style simple narrow, stigma branched.

(2) Disc florets : In the centre of head, bracteate, bisexual, actinomorphic, tubular, pentamerous, epigynous.

Calyx : Absent or pappus.

Corolla : Petals 5, gamopetalous, tubular.

Androecium : 2 stamens, epipetalous, syngenesious, dithecous, bilobed, introrse, filament free.

Gynoecium : Bicarpellary, syncarpous, ovary inferior, unilocular, single ovule in the locule, basal placentation, style single, short, stigma bifid.

(3) Neutral florets : Androecium and gynoecium both are absent. Remaining structures are similar to ray floret and disc florets.

Fruit : Cypsella.

Seed : Exalbuminous.

Floral formula :

Ray florets :

Disc florets :

Neutral florets : \[%\,\text{or}\,\oplus \,{{K}_{\text{0}}}\,{{C}_{\left( 5 \right)}}\,{{A}_{0}}\,{{G}_{0}}\]



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