category taxon characters of sunflower
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The cultivated sunflower is an erect, hardy, often unbranched, coarse, stout-stemmed annual herb, with a varying height up to 4 m. The stem is robust, circular in section, 3-6 cm in diameter, curved below the head, and woody when mature. It is filled with white pith that often becomes hollow with age. The root is a taproot, which can penetrate the soil to a depth of about 3 m, with a large lateral spread of surface roots; however, most of the roots generally remain in the first 50 cm.
Leaves are usually alternate (lower leaves opposite), ovate, cordate, with three main veins, 10-30 cm long, 5-20 cm wide, margin serrate, and carried on long petioles. The colour of the leaves is usually dark green. Lower leaves are larger, broadly ovate or heart-shaped, and attach individually or in pairs. The base of the leaf blade is recurved as it joins a prominent petiole. Teeth on leaf margins range from inconspicuous to clearly present. Upper leaves are smaller, broadly lance-shaped or ovate, and attach individually to the stem.
The flowering head is heliotropic (rotating to face the sun). Flowering heads have 16-30, yellow to gold, ray flowers surrounding a large central disc, and may reach up to 3.5 cm in diameter. Tall, ornamental cultivars have more numerous smaller flowers, whereas cultivars for oil extraction are shorter and have a single flower head. Disc flowers are dark brown to purple. Involucral bracts are broadly ovate, with tips drawn out to a fine point. The disc-shaped flowering head is borne terminally on the main stem, 10-50 cm in diameter, sometimes drooping, and containing 800-8000 bisexual florets. Around the margin of the head there are individual ray flowers, which are sterile, brightly coloured, usually yellow, but varying from deep yellow to red. The brown or purplish disc florets are spirally arranged, flowering from the outer to the centre.
The ovary is inferior with a single basal ovule. Fruits are dry, indehiscent achenes, variable in colour (white, brown, black, or often dark with white stripes). Seed is compressed, flattened oblong, the top truncated and base pointed, 10-25 mm long, 7-15 mm wide. The 1000-kernel weight varies from 50 g to many times this.