4. Brassica rapa L. (field mustard, turnip, bird’s rape)
Pl. 315 j–k; Map
1327
Plants annual or biennial, hairy (at least on the developing leaves) or nearly
glabrous, but not glaucous, the rootstock sometimes thickened and somewhat
fleshy. Stems 20–120(–190) cm long. Basal and lower leaves often absent at
flowering, 15–40(–60) cm long, irregularly pinnately divided or lobed into 3–9
irregularly toothed divisions, petiolate but sometimes with rounded auricles of
tissue at the base, mostly obovate in outline. Stem leaves progressively
reduced toward the tip, the uppermost 2–5 cm long, sessile, the bases clasping
and with rounded auricles, linear to narrowly oblanceolate in outline. Flowers
overtopping the buds. Sepals 3–6 mm long. Petals 0.7–1.0(–1.3) mm long, usually
bright yellow. Fruits (20–)30–80(–110) mm long, ascending or spreading,
circular in cross-section or nearly so, the slender, tapered beak (3–)10–25(–35)
mm long. Seeds 16–30 per fruit, globose, 1.0–1.8 mm in diameter. 2n=20.
April–September.
Introduced, widely scattered in Missouri
(native of Europe, Asia, widely cultivated and commonly escaped in North America). Pastures, margins of crop fields,
roadsides, railroads, and open, disturbed areas.
This species is one of the parents of B. napus, and it can be very
difficult to distinguish from that taxon (see key). Many botanists divide this
species into two varieties. The var. rapa, commonly known as turnip, is
a biennial with a thickened, somewhat fleshy rootstock. In contrast, var. oleifera
DC. (var. campestris (L.) Koch, B. campestris L.) is an annual
with an unthickened taproot. As noted by Al-Shehbaz (1985), these differences
tend not to be apparent in uncultivated plants.