Sunday, August 14, 2022

Uncovering the lair of the sand centipede

I love my wife, but whenever I hike with her my botanizing is reduced to a minimal level.

So when I got the chance to explore a nearby trail on my own, I jumped at the opportunity and took my Nikon 3400 DSLR and  macro lens with me.

The beginning portion of the trail featured a wide sandy path, and on a previous excursion I had noticed really interesting critters that seemed to be slithering under the sand.

The trail covered with "sand centipedes"

It was the usual hot and humid Florida summer day, but this time I literally knelt to observe the little critters. The ground was really hot, but I was entranced by what I saw.

The long bodies of this grass formed sinuous patterns, the leaves half submerged in the sand and looking like the segmented legs of a centipede. I had to be careful not to keep stepping on one, although I knew that they were tough hombres that could take a few adversities and keep on ticking.

A half buried sand centipede as it crawls along the burning ground

Amazingly enough, many of them were in bloom or past bloom, and multitudes of flower and seedheads rose from the ground like the compound eyes of arthropods. They really did look like segmented critters!

Like a stomatopod with 2 eyes peering up from the sand

I identified the species as Eremochloa ophiuroides, which is a turf grass here in the southeast USA. It was introduced to the country in 1916 from China, and these specimens must have "escaped" from nearby lawns. Its common name in the area is "centipedegrass", which either refers to the overall body form or the segmented appearance of the inflorescence.

Inflorescence and stoloniferous culm that had been half buried in the sand

The species looks superficially like Stenotaphrum secundatum (which is called St. Augustine Grass here), but it is smaller and the inflorescence looks quite different.

Interestingly enough, I saw an ant going up and down the flowerheads, and I photographed what looked like aphids perched at the base of the rows of spikelets. The ant seemed to be attracted to the lumpish specks, and I was reminded of some recent research that hypothesized that  Eophiuroides might be pollinated by various insects (Joseph et al, 2020).

Ant cultivating aphids on the inflorescence?

The spikelets themselves were quite small, but the silvery stigma and purple anthers were cute, and I spent some time getting macro shots of the structures. 

Purple anthers and silvery stigmas

I must have spent an hour or more in the heat and sun examining this population of "sand centipedes". I love tiny critters, and the small size of this species was quite attractive. True, it might only be a common turfgrass to some, but the exotic location that I found them in and the fact that they were in bloom captivated the explorer in me. Beauty lies all around us, even in the most common of things.  You just need to open your mind's eye and see.

Literature Cited

V Joseph S, Harris-Shultz K, Jespersen D. Evidence of Pollinators Foraging on Centipedegrass Inflorescences. Insects. 2020 Nov 13;11(11):795. doi: 10.3390/insects11110795. PMID: 33202733; PMCID: PMC7696019.


Thursday, August 11, 2022

Life without Animal Pollinators: Why Grasses Embraced the Wind

Phalaris arundinacea grass inflorescence with white anthers and stigmas and foraging hoverfly.

Some people might be surprised to discover that grasses are flowering plants. That's because when they are compared to the large flowering structures sported by some other angiosperms such as orchids, grass flowers are miniscule indeed. They are not as large and showy as the flowers in some other plants because they are specialized for wind pollination, and thus have no colorful sepals or petals.

This reliance on wind for pollination is called anemophily, and it is in contrast with pollination via insects (entomophily) or vertebrates (zoophily). Around 10-12% of flowering plants are wind-pollinated (Ackerman, 2000), and such plants seem to have evolved specific traits that together make up a "wind pollination syndrome".

Imperata cylindrica inflorescence with purple anthers

Some of the traits in this syndrome are those which grasses exemplify, and include:

  1. Having many pollen grains compared to animal-pollinated plants
  2. Pollen grains are usually unornamented
  3. Having many flowers compared to animal-pollinated plants
  4. Petals are usually small or absent
  5. Nectaries are absent.
  6. Flowers are unscented
  7. Styles and stigma are feathery.
  8. Anthers with the pollen tend to have long filaments and are held away from the leafy structures to aid in wind dispersal
  9. Plants tend to occur in open habitats where wind is plentiful.
Panicum virgatum inflorescence with reddish-brown anthers and pink stigmas

On first look, people might think that wind-pollination is less effective and more "primitive" than pollination by insects and other animals. Indeed, Charles Darwin himself expressed surprise that any plant should display this type of pollination, given how "wasteful" it seemed to him (Darwin, 1876).

But amazingly enough, not only is wind pollination the derived condition (that is, plants started out as being animal-pollinated, but then evolved wind pollination), but studies have shown that anemophily is about as effective as animal pollination when it comes to the percent of pollen that manages to be captured by receptive stigmas! (Harder, 2000).

Andropogon gerardii inflorescence with yellow anthers and purple stigmas

The impetus for the evolution of wind pollination in plants that used to have animals as pollinators has been hypothesized to be situations where animal pollinators become unreliable. For example, some areas that are newly-colonized by a plant may have scarce or absent animal pollinators. In this case, this "pollen limitation" can result in the evolution of wind pollination in the species. Preliminary studies have supported this notion and have shown that pollen limitation is less of a problem in wind pollinated plants than in animal pollinated ones (Friedman and Barrett, 2009).

Paspalum notatum inflorescence with blackish purple stigmas and anthers

In the case of grasses, the very attributes that partly define the family - high density populations that dominate and saturate the environment - may necessitate the use of wind for pollination.  The lack of enough animal pollinators to assure reproduction in these relatively dense populations have pushed the Poaceae to use wind as the primary mechanism for the dispersal of their pollen.

So the next time you get an allergy due to pollen, don't blame the plants. Blame the lack of animal pollinators! ;-)

Cenchrus spp inflorescence with yellow-orange anthers and purple stigmas

References

Ackerman JD. Abiotic pollen and pollination: ecological, functional, and evolutionary perspectives. Plant Systematics and Evolution. 2000;222:167–185.

Darwin C. The effects of cross and self fertilisation in the vegetable kingdom. 2nd edn. London: John Murray; 1876.  

Friedman, J., & Barrett, S. C. (2009). Wind of change: new insights on the ecology and evolution of pollination and mating in wind-pollinated plants. Annals of botany, 103(9), 1515–1527. https://doi.org/10.1093/aob/mcp035

Harder LD. Pollen dispersal and the floral diversity of monocotyledons. In: Wilson KL, Morrison DA, editors. Monocots. Melbourne, Australia: CSIRO; 2000. pp. 243–257.

Thursday, August 4, 2022

The Many Faces of the Inflorescence of Bahia Grass (Paspalum notatum)


Paspalum notatum (aka Bahiagrass) is one of the species used as a perennial turfgrass here in the southeastern USA. It is also a forage grass, and is native to the southern parts of the American continent.

I was walking to the gym last week when I noticed that a lot of the wild grasses along the sides of homes under construction had the very distinctive inflorescence of this species rising up from the ground like little black penants. 

The two spike-like racemes of P. notatum bifurcate from a common point to form the rather attractive digitate flowerhead and seedhead. Each spikelet has two florets in it, one being fertile and the other sterile. 

The immature flowerhead is all green it seems, and as it develops you start seeing the black purple anthers and feathery stigmas (looking like the usual purple caterpillars) coming out of the spikelets.

P. notatum reproduction depends on the ploidy level of the individual plant. Those that are tetraploid (four sets of each chromosome)  are normally apomictic, with unfertilized but viable seeds being produced, while those that are diploid (two sets of chromosomes) are wind-pollinated and cross-pollinate. Seed production is high, and the species is highly successful in its distribution.

After fertilization, the spikelets seem to become darker colored, and after awhile dry out as light brown seeds, which are ready to be dispersed. 

I must admit, I really like this grass. Its flowerheads are distinctive and cute, and as a turfgrass it does not look as coarse and rough as Stenotaphrum secundatum (St.Augustine Grass), which is the other type of grass in our community.