Isotrema is a genus of flowering vines in the family Aristolochiaceae, order Piperales. The genus is closely allied to — and long treated within — the large genus Aristolochia; it has historically been placed as Aristolochia subgenus Siphisia, but molecular and morphological work has supported its recognition as a distinct genus. The defining character of Isotrema is a distinctive three-lobed calyx, which distinguishes it from the bulk of Aristolochia species that bear a one- or two-lipped perianth.
Like other members of Aristolochiaceae, Isotrema species are woody twining vines (lianas) with large, heart-shaped to ovate leaves. The flowers are inflated at the base, narrowing into a curved tube, and opening into three spreading calyx lobes — a shape famously compared to a Dutch smoking pipe, giving members of this group the common names Dutchman's pipe and pipevine. The interior of the flower tube is lined with downward-pointing hairs that temporarily trap pollinating insects (typically small flies or gnats), dusting them with pollen before releasing them, an effective cross-pollination mechanism shared across Aristolochiaceae. Fruit is a dehiscent capsule releasing numerous seeds.
The best-known member of the genus in cultivation is Isotrema macrophyllum (long known as Aristolochia macrophylla), a vigorous deciduous vine native to the eastern United States that has been grown as a garden ornamental for over two centuries. Several species are native to Asia, particularly in China.
Distribution
Isotrema species occur primarily in Asia, with the highest diversity in China, and one well-known species (I. macrophyllum, formerly Aristolochia macrophylla) native to the eastern United States and adjacent Canada (Cumberland and Blue Ridge Mountains, ranging from Maine to Alabama and into Ontario).
Ecology
The inflated, tube-like flowers trap small flies and gnats temporarily inside the perianth tube, which is lined with inward-pointing hairs; these hairs wither once pollination has occurred, releasing the insect. Isotrema macrophyllum is a documented larval host plant for the pipevine swallowtail butterfly (Battus philenor), a relationship that has made it a popular garden plant for wildlife gardeners in eastern North America.
Taxonomy Notes
Isotrema has long been embedded within Aristolochia as subgenus Siphisia, and Wikipedia still presents it this way. GBIF, however, recognizes Isotrema as an accepted genus in its own right. The key morphological marker separating Isotrema from Aristolochia sensu stricto is the three-lobed calyx (vs. a one- or two-lipped perianth in most Aristolochia species).
