Wednesday, September 23, 2009
Family Compositae (Asteraceae) its Examples and Economic Importance
They are nearly all herbs but few are shambby and few reach stature of trees.
Root: Normally taproot, branched and fibrous, root tubers are produced by few species e.g.: Dahlia.
Stem: Soft, erect or prostrate, rarely climbing sometimes woody, usually hairy.
Leaf: Padical or alternate, rarely opposite or whorled, exstipulate, mostly simple of various shapes, smooth, hairy or rough, oil ducts and latex present in some.
Inflorescence: A head or capitulum with few of large number of sessile flowers (ray and disc florets), closely grouped on a common disc like receptacle surrounded by an involucre of few to many, more or less leafy or scaly bracts producing a flower cluster like a single flower in which invloucre is the calyx and the massed coloured flowers represent corolla. Each floret on the disc may bear scale like palea or bristly structure which is regarded as floral bract in which receptacle is said to be floral bract in which receptacle is said to be scaly e.g.: Helianthus, or structure may be lacking and then the receptacle is said to be naked e.g.: Sunchus.
The head of capitulum bears (a) The outer ray florets on the fringe of the receptacle inside the involucre are lingulate and zygomorphic while the rest of the florets the disc florets are all actinomorphic as in sunflower.
(b) All the flowers are lingulate (zygomorphic) and there is distinction between disc florets and ray florets as in sunchus and Dandelion.
(c) All the flowers are tubular and actinomorphic as in Ageratum. In capitulum the ray florets are either female or sterile, the disc florets are usually perfect sometimes unisexual.
Calyx: Reduced to a low ring or a fringe of scaly, bristly or hairy appendage on the rim of the floret axis often persists on the ripe fruit, superior.
Corolla: Sympetalous, 5-lobed or 6 cleft, actinomorphic (in disc florets) or zygomorphic or lingulate (in ray floret).
Androecium: Stamens 5 epipetalous, filaments separate anthers united to form a tube (Syngenious) round the style, dehiscence introse, superior.
Gynoecium: Carpels 2 syncarpous, ovary inferior, unilocular, with one basal ovule, style simple, passing through the staminal tube, stigma bifid with various types of pollen collection hairs or brushes.
Fruit: Achenial, Cypsela, Seed exalbuminous.
Ray florets: +, Nevter, K0 or scales, C(202), A0, G
Disc florets: +, ♀, K0 scales, C(5), A(5), G(2).
e.g.: Sunflower (Helianthus annus), Aster, Lettuce salod (Lactuca), Dandelion, Dahlio, Suncus asper.
(124) FIG 198 PAGE 261 ANGIOSPERM BY G. L. CHOPRA
(1) Root and leaves of cichory leaves of lettuce are used as food.
(2) Flowers of Kusuk yield a dye.
(3) They are used in medicinal purposes.(4) Sunflower seed is used in oil. Tags: Biology , Botany
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