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Published In: The Genera of North American Plants 1: 196–197. 1818. (14 Jul 1818) (Gen. N. Amer. Pl.) Name publication detailView in BotanicusView in Biodiversity Heritage Library
 

Project Name Data (Last Modified On 8/26/2009)
Acceptance : Accepted
 

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38. Thaspium Nutt. (meadow parsnip)

Plants perennial, with fibrous roots. Stems erect or ascending. Leaves alternate and usually also basal (2 to few basal leaves usually present at flowering), the basal and lower stem leaves long-petiolate, the median and upper leaves short-petiolate to nearly sessile, the sheathing bases not or only slightly inflated. Leaf blades ovate to more or less circular in outline, simple or variously lobed or compound. Inflorescences terminal and axillary, compound umbels, short- to long-stalked. Involucre absent or rarely of 1–3 minute, triangular bracts. Rays 6–18, unequal or nearly equal in length. Involucel of 4–9 bractlets or rarely absent, the bractlets shorter than the flower stalks. Flowers 9–19 (to numerous) in each umbellet, all with short stalks (1–4 mm long). Sepals absent or more commonly minute, triangular to obovate scales. Petals obovate, narrowed or tapered abruptly to a short, slender tip, cream-colored to yellow or dark purple. Ovaries glabrous. Fruits ovate to oblong-ovate in outline, flattened (sometimes only slightly) dorsally, glabrous, the mericarps with 5 ribs, these all or nearly all (development on the dorsal and intermediate ribs sometimes irregular) with prominent, somewhat corky, light yellow to straw-colored wings, and prominent reddish brown oil tubes filling the spaces between the ribs. Three species, U.S., Canada.

The genera Thaspium and Zizia are sometimes cited in botany textbooks as an example of genera whose morphologies have converged over time to the point that they are often difficult to distinguish. They may be determined at flowering by the sessile or nearly sessile central flower of each umbellet in Zizia vs. the noticeably short-stalked central flowers in Thaspium, but this character requires practice and patience to discern with confidence. Plants of Zizia also have rootstocks with clusters of slightly to moderately tuberous-thickened roots, whereas the roots of Thaspium species are fibrous and not fleshy or thickened.

Swink and Wilhelm (1994) suggested a small suite of vegetative characters for differentiating species of Thaspium and Zizia: 1) the leaflets in T. barbinode lack a white marginal band and are short-hairy (they call them scaberulous); 2) in Z. aurea, the teeth along the leaf margins are abruptly tapered to relatively blunt tips, whereas those in T. trifoliatum are narrowed or tapered more gradually to the bluntly or sharply pointed tips; 3) the leaf sheaths in T. trifoliatum tend to be slightly more inflated than those of Z. aptera. Although T. barbinode is generally easily separated from the other three Missouri species in these genera, in practice the width of the leaf sheath tends to vary somewhat based on position of the leaf on the stem, whereas the shapes of the marginal teeth vary from the base to the tip of the leaflets, making both of these characters very difficult to interpret in individual plants.

Because classification of the genera of Apiaceae has relied so strongly on characters of fruit morphology, the different fruit types (strongly winged in Thaspium vs. unwinged or only slightly winged in Zizia) have caused some authors to consider the two genera to be relatively distantly related within the tribe Apieae of the subfamily Apioideae. However, Lindsey (1975) questioned whether the genera are truly distinct, based on her examination of a suite of morphological and anatomical characters. Preliminary molecular analyses (Downie et al., 2000) have not fully resolved the genera of Apioideae but have tended to reinforce the hypothesis that Thaspium and Zizia are closely related.

Occasional plants will not key well using the characters presented below. See the treatment of T. trifoliatum for further discussion.

 

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1 1. Basal leaves ternately or ternately then pinnately 2 or 3 times compound, the margins hairy, not whitened ... 1. T. BARBINODE

Thaspium barbinode
2 1. Basal leaves simple or ternately 1 time lobed or compound, the margins glabrous, with a narrow, white border ... 2. T. TRIFOLIATUM Thaspium trifoliatum
 
 
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