Eupatorium hyssopifolium
Eupatorium ×pinnatifidum
hyssop-leaf boneset, hyssop-leaf thoroughwort
thoroughwort
(from short caudices or rhizomes) single, sparsely branched distally, pubescent throughout.
(from short caudices) single, densely branched distally, puberulent throughout.
usually opposite or whorled (distal sometimes alternate, spreading or horizontal); simple, sessile;
blades 3-nerved from bases (laterals sometimes weak), lance-linear, lance-oblong, or linear, 20–60 × 2–15 mm (lengths mostly 6–40 times widths), bases cuneate, margins entire, laciniate-serrate, or serrate, apices acute, faces scabrous (at least abaxial).
opposite (proximal) or alternate (nodes often appearing leafy, lateral buds develop leaves without axis elongation);
sessile;
blades (usually 1–2-pinnately or ternately lobed) or lobes pinnately nerved or 1-nerved, linear, 20–100 × 0.5–1.5 mm, bases narrowly cuneate to attenuate, margins entire or serrate, apices acute, faces puberulent, gland-dotted.
5;
corollas 3–3.5 mm.
7–9;
corollas 2–2.5 mm.
8–10 in 2–3 series, elliptic to oblong, 1.5–5 × 1–1.5 mm, apices obtuse to acute (not mucronate), abaxial faces pubescent throughout (more densely distally).
10–12 in 2–3 series, elliptic to oblong, 1–3.5 × 0.5–1 mm, apices acute to acuminate, shortly mucronate, abaxial faces puberulent, gland-dotted.
in corymbiform arrays.
in subcorymbiform to subpaniculiform arrays.
2–3 mm;
pappi of 20–30 bristles 3.5–4 mm.
1–1.5 mm;
pappi of 20–30 bristles 2.5–3 mm.
= 20.
Eupatorium hyssopifolium
Eupatorium ×pinnatifidum
Varieties 2 (2 in the flora).
Eupatorium hyssopifolium includes both diploid and polyploid cytotypes and presents a complex situation taxonomically. The diploids are placed here in var. hyssopifolium, which also includes polyploids and is characterized by relatively narrow leaves. Variety laciniatum has broader leaves and molecular data suggest that it arose through hybridization between var. hyssopifolium and E. serotinum; the name E. torreyanum has been applied to plants of similar morphology, but these are hybrid derivatives of E. serotinum and E. mohrii. It has also been suggested that var. hyssopifolium has hybridized with E. album var. album to form E. saltuense, and with diploids of E. linearifolium to form polyploid populations included here under E. linearifolium. Eupatorium hyssopifolium is characterized by a tendency for the leaves to be whorled, in 3s and 4s, and for axillary buds to produce leaves without expansion of shoots; the combination of whorled leaves and suppressed axillary shoots gives the nodes a somewhat tufted or verticillate appearance.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Eupatorium ×pinnatifidum has been suggested to represent a series of recurrent hybrids between E. capillifolium or E. compositifolium and E. perfoliatum; molecular data, in addition to confirming that this combination occurs, also show that E. serotinum rather than E. perfoliatum is the second parent in at least some plants included in E. ×pinnatifidum.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
1. Leaves mostly linear, 2–5 mm wide, margins usually entire or obscurely serrulate | var. hyssopifolium |
1. Leaves mostly lance-linear to lanceolate, 5–15 mm wide, margins usually laciniately toothed | var. laciniatum |
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