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Although the most remarkable of the plants which constitute this genus are found in tropical countries, some half a dozen woody, climbing species can be grown in the open air in Britain which present so remarkable a flower-structure and are, withal, so vigorous in growth, that one or more of them ought to be seen in every garden. Leaves alternate, mostly heart-shaped. The flower has no corolla; the calyx (or perianth) is more or less tubular, curiously inflated, and bent so as to resemble a siphon or Dutch pipe. Some of the flowers of tropical aristolochias are fly-traps; the insect is attracted by a foetid odour, and enters the tube, which is clothed with hairs pointing downwards; these hairs offer no obstacle to the ingress of the fly, but effectually bar its return.
The hardy species like a good loamy soil, and can be increased by division or by cuttings. They are suitable for the various positions adapted for climbers.
All the species described, with the exception of A. altissima and the related A. sempervirens, belong to a group which by some botanists is separated from Aristolochia as the genus Isotrema. The relevant synonyms (which were inserted in the second printing) are:
A. californica Torr. Isotrema californicum (Torr.) H. Huber.
A. chrysops (Stapf) Wils. Isotrema chrysops Stapf.
A. macrophylla Lam. Isotrema sipho (L’Hérit.) Raf.; I. durius (Hill) H. Huber; I. macrophyllum (Lam.) C. F. Reed.
A. moupinensis Franch. Isotrema moupinense (Franch.) Stapf MS, inedit.
A. tomentosa Sims Isotrema tomentosum (Sims) H. Huber.