(It’s twirly) (too early)
Fabaceae, pea family
Although John and I have an early field trip planned for tomorrow (Friday), the rest of the day and evening will be unavailable for blog writing, so jumping the gun a day early.. A couple “new” non-native weeds, both with a twist, have popped up hereabouts in Jupiter, Florida.
Fourspike heliotrope (Euploca procumbens) invading from the American Tropics is perhaps welcomed northward by Global Warming? A coiled inflorescence helps with recognition of this and other heliotropes.
Twisted weed Euploca procumbens. Compare its snail shell flower cluster on the left with the keel tip below.
Possessing its own much smaller coil, literally growing alongside the Heliotrope, is Leptospron adenanthum, which is one of several snail-flowered legumes.
We could call them “corkscrew vines” or “snail flower vines,” as we could some other legume species, because they have curling in the flower reminiscent of a snail shell. Several not-closely-related members of the Pea and Bean Family do the twist, three of them lookalike garden flowers distinguished in the notes below. Any of these “snail flower vines” encountered in Florida are escapes from ornamental horticulture or conceivably from cultivation as fodder. They are in different genera, which begs the question, “why do distantly related species all evolve independently the same weird contortions?” There must be something useful to it, some common benefit. Convergent evolution.
Dangling from tree branches in South Florida, including Cypress Creek Natural Area in Jupiter, is Leptospron adenanthum, an example of how Mother Nature can take an old groundplan and give it a new twist. We need to understand the straight basic structure of a pea-type flower as a staring point. Then we’ll put a new spin on it.
In most pea-type flowers there are 5 petals: a banner petal behind the rest of the flower like a photographer’s backdrop; two wing petals reaching out on either side like stubby arms; and a central keel made of two petals pressed face-to-face to form a single envelope. The keel resembles a very narrow boat sealed on three sides and open or partly so at the top. The pollen-making anthers and pollen-receiving stigma lie hidden within the keel where visiting insects land. The visitor’s weight pushes the keel down, allowing the anthers and stigma to pop up via the open top and touch the underside of the insect.
Leptospron turns the beat around You have all these petals, but in novel and complex arrangements with altered functions. The biggest alteration is to the keel. It becomes long, tubular, curled, and coiled. The keel curls in its lower half to form an arched doorway covering the passageway into the flower. Then it goes through multiple corkscrew twists toward the tip, there resembling a small snail shell. The coiled shell is positioned near the central base of the entranceway, has a yellow tip presumably attractive to a bee, and has emerging from the tip the stigma and anthers, the sexual business parts of the flower.
The banner has a yellowish spot near its base along the same line of sight as the all-critical yellow keel tip. The two yellowish spots team up to make an emphatic double bull’s eye—bee aim here!
One wing petal takes over as landing platform, leading compellingly to the dual yellowish spots. This all guides the right bee into receiving and delivering pollen with precision.
To be annoyingly redundant, the odd modifications include:
- Having a wing become the landing platform with nectar guides into the flower
- Having the keel arch up over the top of the entrance giving rigidity and definition to the approach. (And maybe the bee’s contact with the arch helps with pollen placement or release.)
- Having the distal keel portion coiled to place and pick up pollen surgically at its tip.
- Having the banner offer yellowish reinforcement to the small yellow zone on the keel tip.
Night is odd too. The entrance closes. The wing petal not serving as the main landing platform (on our right viewing the flower) folds upward and blocks the arch entranceway. The wing petal (on our left) serving by day as the landing platform seems to rise and push itself against the flat face of the coiled keel tip, hiding its yellow marking, and blocking access. The flower is closed for business!
Notes for over-achievers.
A few details beyond the main story. Botanists have suggested the ultra-long keel may filter out “wrong” pollen. To achieve sperm delivery a pollen grain on the stigma must produce a threadlike pollen tube that grows a microscopic thread, the pollen tube, down through the style to deliver sperm to the eggs in the immature seeds. Pollen tube, style, and keel are all the same length. Only the correct pollen tubes can go the extra distance. Those from other species come up short and thus fail to mis-fertilize the eggs..
Tthe anthers and stigma juxtaposed in that tight curling keel risk pollen transfer from anther to stigma within the keel. But no worries. Leptospron has a membrane protecting the stigma. Probably an aggressive bee breaks the membrane.
*Three showy legumes with similar “snail” flowers. These resemble each other and are mututally confused. The guide below might help. As far as I’m concerned, English names for any of them are almost worthless.
Cochlisanthus caracalla (Vigna caracalla, Phaseolus caracalla)
Whole flower twisted or wavy, and the buds twisted (vs. buds straight in the others)
Flowers numerous in dangling clusters (vs. flowers few or one)
Pod resembles green bean (vs. flat)
There is a detailed study of tis species in Ann Bot. 2008 102(3): 305–316.
Leptospron adenanthum (Vigna adenantha)
Buds not twisted (vs. twisted in Cochlisanthus)
Flowers few or just one per cluster (vs numerous in Cochlisanthus)
Keel with about 3 full twists (vs. merely looped into nearly a circle in Sigmoidotropis)
Fruit flat and uniquely C-shaped, much broader than that of Sigmoidotropis
Sigmoidotropis speciosa (may be sold as Phaseolus giganteus)
(Often called “Giant Snail Bean)
Keel looped nearly into a circle (not with multiple corkscrew twists as in Leptospron and Cochlisanthus)
Fruit flat, long, narrow, straight, the halves twisting upon opening (much longer and narrower than in Leptospron)