Description
Verbascum thapsus is a dicotyledonous plant that produces a rosette of leaves in its first year of growth. The leaves are large, up to 50 cm long. The second year plants normally produce a single unbranched stem usually 1–2 m tall. In the East of its range in China, it is, however, only reported to grow up to 1.5 m tall. The tall pole-like stems end in a dense spike of flowers that can occupy up to half the stem length. All parts of the plants are covered with star-shaped trichomes. This cover is particularly thick on the leaves, giving them a silvery appearance. The species' chromosome number is 2n = 36.
On flowering plants the leaves are alternately arranged up the stem. They are thick and decurrent, with much variation in leaf shape between the upper and lower leaves on the stem, ranging from oblong to oblanceolate, and reaching sizes up to 50 cm long and 14 cm across (19 inches long and 5 inches wide). They become smaller higher up the stem, and less strongly decurrent down the stem. The flowering stem is solid and 2–2.5 cm (nearly an inch) across, and occasionally branched just below the inflorescence, usually following damage. After flowering and seed release the stem and fruits usually persist in winter, drying into dark brown, stiff structures of densely packed, ovoid-shaped and dry seed capsules. The dried stems may persist into the following spring or even the next summer. The plant produces a shallow taproot.
Flowers are pentamerous with (usually) five stamen, a 5-lobed calyx tube and a 5-petalled corolla, the latter bright yellow and an 1.5–3 cm (0.5–1 inch) wide. The flowers are almost sessile, with very short pedicels (2 mm, 0.08 in). The five stamens are of two types, with the three upper stamens being shorter, their filaments covered by yellow or whitish hairs, and having smaller anthers, while the lower two stamens have glabrous filaments and larger anthers. The plant produces small ovoid (6 mm, 0.24 in) capsules that split open by way of two valves, each capsule containing large numbers of minute brown seeds less than a millimetre (0.04 in) in size, marked with longitudinal ridges. A white-flowered form, V. thapsus f. candicans, is known to occur. Flowering lasts for up to three months from early to late summer (June to August in northern Europe), with flowering starting at the bottom of the spike and progressing irregularly upward; each flower opens for part of a day and only a few open at the same time around the stem.
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