The “Seedbox” Ludwigias in winter.

If you live in the eastern half of the United States, you may know and love “Seedbox,” Ludwigia alternifolia, with its bright yellow four-petal flower and its intriguing cube-shaped seed capsule that persists through fall to form a classic part of the winter landscape.

If you’re in the coastal-plain fringes of the southeastern states, though, things are more complicated. There you have no fewer than four different seedbox-forming Ludwigias, all with four-petal yellow flowers, and all with cube-shaped seed capsules that last through winter. Some details of flower and leaf structure distinguish them in the blooming season, but what about over winter?

(The ranges of the four seedbox Ludwigias are shown at
(http://bonap.net/Napa/TaxonMaps/Genus/County/Ludwigia). Look for L. alternifolia, L. hirtella, L. maritima and L. virgata.)

Ludwigia alternifolia, the widespread Seedbox, is a profusely branched plant with many seed capsules, each borne on a pedicel that is shorter than the capsule (the other species have longer pedicels). Each box-like capsule has a round pore in its top, surrounded by four flat nectary discs. The tiny seeds escape through the pore or are dropped when the capsule disintegrates. The combination of branching habit, short pedicels and flat nectary discs on the box-like seed capsule is usually sufficient to identify L. alternifolia in winter, even where other seedbox Ludwigias are present.

A second species, Ludwigia hirtella, Spindleroot, has a seedbox and stem covered with conspicuous long hairs that make it hard to confuse with any of the other seedbox Ludwigias. Spindleroot grows in flatwoods and bogs of the coastal plain from Virginia to Texas.

The real identification problem concerns the other two seedbox Ludwigias, L. maritima (Seaside Primrose-Willow) and L. virgata (Savanna Primrose-Willow), found near the coast from North Carolina to Mississippi and on the Florida peninsula. Both have smooth, box-shaped seed capsules borne on wandlike, relatively unbranched stems. In contrast to L. alternifolia, both have plump nectary discs that appear as lumps around the pore at the top of the seed capsule. As the range of these two species nearly coincides, and they are not clearly differentiated by habitat, I wanted to determine if there are characters that could be used to distinguish these species’ winter seedboxes.

Ludwigia maritima was first described as a species by Harper in 1904. He distinguished it from L. virgata on the basis of several characters, most of which were in the flower, but he described the seed capsule of L. virgata as “very slightly winged on the angles” and the capsule of L. maritima as “distinctly winged on the angles.” He also included a comparative sketch with showed the capsule of L. maritima as larger and more inflated than in L. virgata (Torreya 4:161-4).

Capsule size difference is mentioned in two widely used works. Manual of the Vascular Flora of the Carolinas (1964, Radford, Ahles, and Bell) notes that L. virgata’s capsules are “5-7 mm long, 3.5-5 mm broad” and the species is found in “bogs and low savannahs,” whereas L. maritima’s capsules are “6-10 mm long, 4.5-7 mm broad” with the species growing in “savannahs, ditches and low pinelands.” Both are described as cubical, 4-angled, and narrowly winged. Godfrey and Wooten (1981, Aquatic and Wetland Plants of the Southeastern United States) described the capsule of L. virgata as “hard, 5-7 mm long, 3.5-5 mm broad” with seeds 0.5-0.8 mm long, compared to L. maritima capsules “tending to fracture easily, 6-10 mm long, 4.5-8 mm broad, seeds usually not exceeding 0.5 mm long,” with habitats for the two species similar except that in addition to wetlands, L. virgata is found “not infrequently on well-drained sandy pinelands.”

So seed capsules of L. maritima are said to be taller, broader, and more distinctly winged on the angles than capsules of L. virgata. Can these characters be used for winter identification?

I had made a number of observations of both species to iNaturalist.org during the flowering season, when they were relatively easy to identify. In late December, I returned to the site of observations I’d made in Jackson County, Mississippi, to look for winter capsules, and succeeded in finding three stalks of each species that contained seed capsules. I measured 7 capsules on each plant (height including nectary discs, breadth) as well as the capsule pedicel length, and l looked for other characters that might help in winter identification.

The capsules of L. maritima were noticeably and significantly larger than those of L. virgata, with a small amount of overlap (Table 1). In general, L. maritima capsules had dimensions greater than 6.5mm x 5 mm, and L. virgata capsules measured less than 6 mm x 5mm. Although this may seem like a small difference, it was readily apparent when the capsules were side by side. There was no difference in pedicel length. Only one of the six plants had distinctly winged capsules, so that feature was not particularly helpful in distinguishing species.

Table 1. Capsule measurements
Ludwigia maritima

height (mm) width (mm)
mean 7.04 5.33
stdev 0.42 0.38
N 21 21

Ludwigia virgata

height (mm) width (mm)
mean 5.58 4.20
stdev 0.44 0.55
N 21 21

The capsules of the two species were also shaped differently, with the larger L. maritima capsules more bowed out on the sides like a barrel. When viewed from the top, the nectary discs of the smaller L. virgata capsules appeared to take up more of the top surface, and each disc was ringed by silvery hairs not seen on the discs of L. maritima.

Godfrey and Wooten’s observation that the small L. virgata capsule is “hard” and the larger L. maritima capsule “tends to fracture easily” was also borne out during handling of the seed capsules.

I conclude that it should be possible to distinguish the two Ludwigias from their winter capsules alone if they can be measured, and that they should also be able to be distinguished in photographs where the nectary discs are clearly visible and there is a good sense of scale.

The six plants used in this investigation:

https://www.inaturalist.org/observations/67428016
https://www.inaturalist.org/observations/67428011
https://www.inaturalist.org/observations/67428009
https://www.inaturalist.org/observations/67428015
https://www.inaturalist.org/observations/67428014
https://www.inaturalist.org/observations/67428010

Publicado por janetwright janetwright, 02 de enero de 2021

Comentarios

This is a wonderful, thorough study! I appreciate your work very much.

Publicado por suz hace 10 meses (Marca)

Wow! Thank you for the detailed look at these species.

Publicado por joshuahodge hace 10 meses (Marca)

Excellent info.

Publicado por bjkauffman hace 10 meses (Marca)

This is awesome. Thanks for the info!

Publicado por glsasser hace 10 meses (Marca)

Very interesting! Was not aware of their existence before I read this post. Thank you.

Publicado por yarddoc hace 10 meses (Marca)

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