Valentine’s Day may have passed, but it is still evidenced by the discounted chocolate and wilting roses in the shop windows, and the childhood rhyme “Roses are red, violets are blue…” that is echoing incessantly in my head. The repeated rhyme has gotten me thinking: of all the things about Viola to remark on, their sometimes blue colour is probably their least remarkable quality!
Why are violets the perfect token of affection for your favourite biologist? It’s all about reproduction. Viola species can produce showy flowers worthy of a Valentine’s gift that range in colour from blue to purple, white or yellow. These flowers are known as chasmogamous, a term derived from the greek words for ‘open marriage’. They are open to the world and any available violet in it, allowing for cross pollination as insects and the wind carry pollen between different Viola flowers. However, some violet species also produce a second type of flowers underground that are pale and never open their petals. These cleistogamous (‘closed marriage’) flowers are self-pollinated and receive no genetic input from other individuals. Eventually, the mature seeds of these cleistogamous flowers will be released directly into the soil, or near the soil surface. This production of cleistogamous flowers is thought to be a ‘backup method’ to ensure successful seed production even when pollinators are scarce and seed and flower predation are high.
It’s not just the production of hidden flowers that makes violets unique. The genus also has evolved two dispersal methods to ensure that its seeds find new territories to colonize. Most species have a form of ballistic dispersal, whereby contraction of the seed pod as it dries eventually shoots the seeds up to several metres away from the mother plant.
Those violets that do not have a ballistic dispersal mechanism have evolved instead a complex, mutually beneficial partnership with ants, a phenomenon called myrmechory. The ants act as seed carriers, depositing the Viola seeds in new locations around the forest, but not before they have collected payment by gnawing off the fatty, lipid rich protuberance at the base of the seeds called an elaiosome. Myrmechorous Viola species have drooping flowers on weak stems that bend to the ground under the weight of the developing seed capsule. When the seed capsule finally opens, it spills the seeds out into a pile on the soil, rather than flinging them forcibly from the mother plant. The seeds produce a compound called 1,2-diolein that acts as a signal to foraging ants. Attracted by the smell of the seeds, the ants collect them and take them back to the nest, where larvae are fed the elaiosomes, and the seeds are then deposited in refuse piles at the nest entrances or the borders of the ants’ territory where they eventually germinate. The relationship between the violets and ants is finely tuned. The plant has evolved to release its seeds early in the season, before the peak in abundance of other, more attractive food sources for the ants, and the seed capsules open during the period of highest ant activity, which coincides with the lowest levels of activity of seed predators like rodents. Violets benefit in a number of secondary ways from ant dispersal too. The chewing behaviour of the ants can actually break the dormancy of the seeds, improving their germination rates, and the refuse piles the ants leave the seeds in represent nutrient enriched microsites that are favourable for growth. Violets may be blue, but it is their reproductive biology that makes them fascinating!