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.