Kyllinga odorata (Cyperaceae)
Ludwigia species (Onagraceae)
Today John and George sweated through Halpatioke Park in Stuart, Florida. With resort-quality lakes deep in the forest, Halpatioke is always a beautiful source of botanical surprises from “northern” species near their southern limit, such as Partrideberry (Mitchella repens) to “tropical” species near their northern border, such as Lax Panicum (Steinchisma laxum). But what made an impression today were the weird flowers—the ones not playing by the usual rules.
First, in the lawn there was the native Fragrant Kyllinga (Kyllinga odorata). We all know the vast majority of grasses and sedges to be wind-pollinated, no showy flowers and no floral fragrance. Florida wildflower enthusiasts know an exception to this to be the species of Rhynchspora called Painted Sedges, with white bracts at the top simulating petals. (Some may have learned these under the name Dichromena, two-toned.) We are not talking about those today.
Fragrant Kyllinga is an inconspicuous little sedge, often a turf weed, providing a less familiar case of an insect-pollinated sedge. In most Kyllingas the flower cluster is green, odorless and typically sedgelike. But in Fragrant Kyllinga the thimble-shaped flower cluster is nearly white and sweetly fragrant. Evolution works in weird ways. Wind-pollinated plants are generally regarded as having abandoned creature-mediated pollination. Exceptional fragrant or colorful plants in a wind-pollinated family thus seem to have “reinvented” insect pollination. With petals gone, the white color must be on modified leaves under the flowers in the Painted Sedges and Fragrant Kyllinga.
Not long after sniffing the Kyllinga we came upon a lake shore patch of what we took to be Small-Fruited Ludwigia, Ludwigia microcarpa, another head-scratcher. Most of us probably tend to think of Ludwigias as having conspicuous yellow blossoms, such as the showy Peruvian Primrose-Willow rising bright yellow from roadside ditches. Yet a number of Ludwigia species have done away with petals or nearly so. (What look like petalsin hte photos below are sepals.) Examples of petal-less Ludwigias (by JB) are illustrated below:
These obviously differ dramatically from the yellow-petal Ludwigias. Maintaining any unnecessary structure is a biological liability and waste of energy, sort of like maintaining an unused residential swimming pool, but petals not needed?
Are those petal-less species self-pollinated, or able to make seeds without pollination? Those abilities are not rare in the plant world, but I doubt that explains the absence of petals. The flowers without petals have otherwise well-formed open flowers, the sepals have a slightly petal-like appearance, even becoming creamy or tinted rather than the usual sepal-green. The flower centers can be colorful as well.
Now we go to pure speculation. Here is a guess. Maybe the genus has divided its pollination between bees drawn to the big bright petals on some species, and other insect visitors not particularly drawn to big yellow petals. Losing petals genetically is probably a minor change, basically “instant evolution.” What’s striking is that the petal-less species are not all most closely related to each other. Some have species with petals as their closest relatives, implying that that petals were lost more than once as separate events. There must indeed be something “good” about petal loss.
If I can beg your credibility a little, the petal-free Ludwigia flowers are not terribly different from flowers of roughly similar sizes and colorations encountered in many other plant types in similar marshy or shoreline habitats, for instance, Swamp Hornpod (Mitreola sessilifolia), Herb-of-Grace (Bacopa monnieri), Buttonweeds (Diodia species), Bartonia (Bartonia verna), Bedstraws (Galium species), Water Pimpernels (Samolus species), and others.
Below are several species not related to Ludwigia. The petal-less Ludwigias are more similar to these than they are to their yellow-flowers relatives. They are all roughly in the same size range.