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Excerpts from Jim Conrad's
Naturalist Newsletter
entry from field notes dated January 17, 2023, taken on trail up the southeast-facing slope of Peña de Bernal on the northwestern side of Bernal, elevation ±2,220m (7280 ft); bedrock of intrusive, igneous dacite rock, similar to granite; Querétaro state, MÉXICO, (N20.748°, W99.949°)
SNAIL VINE
At the very edge of the high, northwestern side of the town of Bernal, along the much-trodden foot trail leading to the pay booth and turntables at the entry to the trail climbing the southeast face of Peña de Bernal, a great mass of mutually entangling vines dangled from an overhead framework partly sheltering the trail. On a certain vine, the above blossom was one of several blossoming, even at this very dry time of the year.
Clearly the blossom is a "papilionaceous," or butterfly-like, flower of the big Bean Family, the Fabaceae. Note how it consists of a large top petal known as the standard or banner, two side petals called wings, and two lower petals fused along their common margin to form a scoop-shaped structure known as the keel, like the keel of a boat. The striking feature of this flower is that its keel is spectacularly long and slender, and coiled like a corkscrew. Here's a view of the blossom from the side:
We've seen a flowering vine very similar to this, the Sigmoidotropis elegans, growing weedily along a road in Maya territory back in the Yucatan. However, that vine's flowers seem a little different from these, as with the top petal's color. More details on this Peña de Bernal plant were needed for a solid ID. For instance, in the Bean Family, for identification purposes, often the leafy stipules at petiole bases are distinctive. This vine's stipules were broad and rounded at their tips:
The main flowering season seems to have been passed for this vine, because many more legume-type fruits appeared than flowers.
In fact, in places the overhanging vine mass produced a very heavy crop of pods:
This robust vine, which in full blossom must have been spectacular, is an ornamental favorite with no accepted binomial -- no scientific name. In commercial markets it's known by such English names as Snail Vine, Snail Bean, Corkscrew Vine, and more, plus it's often labeled with the formal-looking name of Phaseolus giganteus. However, the name Phaseolus giganteus has never been properly published and is not recognized by standard taxonomic references. It's unusual for such a popular ornamental plant to lack any kind of accepted technical name, but that appears to be the case here.
The situation is further confused by there being another taxon also sold under the above-mentioned English names, but it is recognized by the valid taxonomic binomial of Cochliasanthus caracalla. The two taxas' leaves and stems look and behave exactly the same, but the flowers are very different. I've been unable to find any authoritative statement clarifying the situation, other than the Wikipedia page at this time suggesting, with no citation, that the vine with flowers like our Peña de Bernal plant "appears to be Vigna speciosa or a close relative."