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.