The Hidden Flowers of Viola

Violas keep a secret hidden below their foliage. Sometimes they even bury it shallowly in the soil near their roots. I suppose it’s not a secret really, just something out of sight. There isn’t a reason to show it off, after all. Showy flowers are showy for the sole purpose of attracting pollinators. If pollinators are unnecessary, there is no reason for showy flowers, or to even show your flowers at all. That’s the story behind the cleistogamous flowers of violas. They are a secret only because unless you know to look for them, you would have no idea they were there at all.

Cleistogamy means closed marriage, and it describes a self-pollinating flower whose petals remain sealed shut. The opposite of cleistogamy is chasmogamy (open marriage). Most of the flowers we are familiar with are chasmogamous. They open and expose their sex parts in order to allow for cross-pollination (self-pollination can also occur in such flowers). Violas have chasmogamous flowers too. They are the familiar five-petaled flowers raised up on slender stalks above the green foliage. Cross-pollination occurs in these flowers, and seed-bearing fruits are the result. Perhaps as a way to ensure reproduction, violas also produce cleistogamous flowers, buried below their leaves.

an illustration of the cleistogamous flower of Viola sylvatica opened to reveal its sex parts — via wikimedia commons

Flowers are expensive things to make, especially when the goal is to attract pollinators. Colorful petals, nectar, nutritious pollen, and other features that help advertise to potential pollinators all require significant resources. All this effort is worth it when it results in the ample production of viable seeds, but what if it doesn’t? Having a method for self-pollination ensures that reproduction will proceed in the absence of pollinators or in the event that floral visitors don’t get the job done. A downside, of course, is that a seed produced via self-pollination is essentially a clone of the parent plant. There will be no mixing of genes with other individuals. This isn’t necessarily bad, at least in the short term, but it has its downsides. A good strategy is a mixture of both cross- and self-pollination – a strategy that violas employ.

The cleistogamous flowers of violas generally appear in the summer or fall, after the chasmogamous flowers have done their thing. The fruits they form split open when mature and deposit their seeds directly below the parent plant. Some are also carried away by ants and dispersed to new locations. Seeds produced in these hidden flowers are generally superior and more abundant compared to those produced by their showy counterparts. People who find violas to be a troublesome lawn weed – expanding far and wide to the exclusion of turfgrass – have these hidden flowers to blame.

That being said, there is a defense for violas. In the book The Living Landscape by Rick Darke and Doug Tallamy, Tallamy writes: “Plants such as the common blue violet (Viola sororia), long dismissed by gardeners as a weed, can be reconstituted as desirable components of the herbaceous layer when their ecosystem functionality is re-evaluated. Violets are the sole larval food source for fritillary butterflies. Eliminating violets eliminates fritillaries, but finding ways to incorporate violets in garden design supports fritillaries.”

sweet violet (Viola odorata)

In my search for the cleistogamous flowers of viola, I dug up a sweet violet (Viola odorata). I was too late to catch it in bloom, but the product of its flowers – round, purple, fuzzy fruits – were revealed as I uprooted the plant. Some of the fruits were already opening, exposing shiny, light brown seeds with prominent, white elaiosomes, there to tempt ants into aiding in their dispersal. I may have missed getting to see what John Eastman calls “violet’s most important flowers,” but the product of these flowers was certainly worth the effort.

Fruits formed from the cleistogamous flowers of sweet violet (Viola odorata)

Up close and personal with the fruit of a cleistogamous flower

The seeds (elaiosomes included) produced by the cleistogamous flower of sweet violet (Viola odorata)

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Beavers and Water Lilies – An Introduction to Zoochory

Beavers are classic examples of ecosystem engineers. It is difficult to think of an animal – apart from humans – whose day-to-day activities have more impact on the landscape than beavers. Their dam building activities create wetlands that are used by numerous other species, and their selective harvesting of preferred trees affects species composition in riparian areas. And that’s just the start. Their extensive evolutionary history and once widespread distribution has made them major players in the landscape for millions of years.

Today, the beaver family (Castoridae) consists of just two extant species: Castor fiber (native to Eurasia) and Castor canadensis (native to North America). Both species were hunted by humans to the brink of extinction but, thanks to conservation efforts, enjoy stable populations despite having been eliminated from much of their historical ranges. Before the arrival of Europeans, North American beavers are estimated to have been anywhere from 60 million to 400 million strong. Extensive trapping reduced the population to less than half a million. Today, 10 million or more make their homes in rivers, streams, and wetlands across the continent.

North American beaver (Castor canadensis) - photo credit: wikimedia commons

North American beaver (Castor canadensis) – photo credit: wikimedia commons

Beavers are herbivores, and they harvest trees and shrubs to build dams and lodges. Their interactions with plants are legion, and so what better way to introduce the concept of animal-mediated seed dispersal than beavers. Plants have several strategies for moving their seeds around. Wind and gravity are popular approaches, and water is commonly used by plants both aquatic and terrestrial. Partnering with animals, however, is by far the most compelling method. This strategy is called zoochory.

Zoochory has many facets. Two major distinctions are epizoochory and endozoochory. In epizoochory, seeds become attached in some form or fashion to the outside of an animal. The animal unwittingly picks up, transports, and deposits the seeds. The fruits of such seeds are equipped with hooks, spines, barbs, or stiff hairs that help facilitate attachment to an animal’s fur, feathers, or skin. A well known example of this is the genus Arctium. Commonly known as burdock, the fruits in this genus are called burs – essentially small, round balls covered in a series of hooks. Anyone who has walked through – or has had a pet walk through – a patch of burdocks with mature seed heads knows what a nuisance these plants can be. But their strategy is effective.

The burs of Arctium - photo credit: wikimedia commons

The burs of Arctium – photo credit: wikimedia commons

Endozoochory is less passive. Seeds that are dispersed this way are usually surrounded by fleshy, nutritious fruits desired by animals. The fruits are consumed, and the undigested seeds exit out the other end of the animal with a bit of fertilizer. Certain seeds require passage through an animal’s gut in order to germinate, relying on chemicals produced during the digestion process to help break dormancy. Other seeds contain mild laxatives in their seed coats, resulting in an unscathed passage through the animal and a quick deposit. Some plants have developed mutualistic relationships with specific groups of animals regarding seed dispersal by frugivory. When these animal species disappear, the plants are left without the means to disperse their seeds, which threatens their future survival.

Beavers rely on woody vegetation to get them through the winter, but in warmer months, when herbaceous aquatic vegetation is abundant, such plants become their preferred food source. Water lilies are one of their favorite foods, and through both consumption of the water lilies and construction of wetland habitats, beavers help support water lily populations. This is how John Eastman puts it in The Book of Swamp and Bog: “Beavers relish [water lilies], sometimes storing the rhizomes. Their damming activities create water lily habitat, and they widely disperse the plants by dropping rhizome fragments hither and yon.”

Fragrant water lily (Nympaea odorata) - photo credit: wikimedia commons

Fragrant water lily (Nymphaea odorata) – photo credit: wikimedia commons

The seeds of water lilies (plants in the family Nymphaceae) are generally dispersed by water. Most species (except those in the genera Nuphar and Barclaya) have a fleshy growth around their seeds called an aril that helps them float. Over time the aril becomes waterlogged and begins to disintegrate. At that point, the seed sinks to the bottom of the lake or pond where it germinates in the sediment. The seeds are also eaten by birds and aquatic animals, including beavers. The aril is digestible, but the seed is not.

In her book, Once They Were Hats, Frances Backhouse writes about the relationship between beavers and water lilies. She visits a lake where beavers had long been absent, but were later reintroduced. She noted changes in the vegetation due to beaver activity – water lilies being only one of many plant species impacted.

Every year in late summer, the beavers devoured the seed capsules [of water lilies], digested their soft outer rinds and excreted the ripe undamaged seeds into the lake. Meanwhile, as they dredged mud from the botom of the lake for their construction projects, they were unintentionally preparing the seed bed. Seeing the lilies reminded me that beavers also inadvertantly propagate willows and certain other woody plants. When beavers imbed uneaten sticks into dams or lodges or leave them lying on moist soil, the cuttings sometimes sprout roots and grow.

Other facets of zoochory include animals hoarding fruits and seeds to be eaten later and then not getting back to them, or seeds producing fleshy growths that ants love called elaiosomes, resulting in seed dispersal by ants. Animals and plants are constantly interacting in so many ways. Zoochory is just one way plants use animals and animals use plants, passively or otherwise. These relationships have a long history, and each one of them is worth exploring and celebrating.

Harvester Ants – Seed Predators and Seed Dispersers

“The abundance of ants is legendary. A worker is less than one-millionth the size of a human being, yet ants taken collectively rival people as dominant organisms on the land. …  When combined, all ants in the world taken together weigh about as much as all human beings.” – Journey to the Ants by Bert Hölldobler and Edward O. Wilson

Considering how abundant and widely distributed ants are, it is easy to imagine the profound role they might play in the ecosystems of which they are a part. In fact, in the epilogue to Hölldobler and Wilson’s popular book about ants (quoted above), they conclude that in a world without ants, “species extinction would increase even more over the present rate, and the land ecosystems would shrivel more rapidly as the considerable services provided by these insects were pulled away.” It is no doubt then that ants, through their myriad interactions with their surroundings, are key players in terrestrial ecosystems.

photo credit: www.eol.org

photo credit: www.eol.org

Harvester ants offer a prime example of the important roles that ants can play. In the process of collecting seeds for consumption, harvester ants can help shape the abundance and distribution of the plants in their immediate environment. They do this by selecting the types and amounts of seeds they collect, by abandoning seeds along their collection routes, and by leaving viable seeds to germinate in and around their nests. Hölldobler and Wilson have this to say about harvester ants:

[The] numerical success [of ants] has allowed them to alter not just their nest environments, but the entire habitats in which they live. Harvesting ants, species that regularly include seeds in their diet, have an especially high impact. They consume a large percentage of the seeds produced by plants of many kinds in nearly all terrestrial habitats, from dense tropical forests to deserts. Their influence is not wholly negative. The mistakes they make by losing seeds along the way also disperse plants and compensate at least in part for the damage caused by their predation.

There are more than 150 species of harvester ants, spanning at least 18 genera. They are found throughout the world (except extreme cold locales) and are particularly common in arid to semi-arid environments. Pogonomyrmex is one the largest genera of harvester ants with nearly 70 species occurring throughout North, Central, and South America. Messor is another large genus of harvester ant species that mainly occurs in Europe, Asia, and Africa. Both genera build large nests and move massive amounts of soil in the process.

Seed dispersal by harvester ants (also known as diszoochory) is a type of secondary (or Phase II) seed dispersal. It is a case of serendipity, as the dispersal occurs largely by accident. Some plants, on the other hand, have developed a mutualistic relationship with ants, enlisting them to disperse their seeds by way of an elaiosome – a fleshy, nutritious structure attached to seeds that attracts ants. Seeds with such structures are picked up by ants and brought to their nests where the elaiosome is consumed and the seed is left to germinate. This form of ant-mediated dispersal is called myrmecochory and is typically not carried out by harvester ants.

photo credit: wikimedia commons

photo credit: wikimedia commons

Harvester ant colonies have both direct and indirect influences on their surrounding environments; however, there is a dearth of research elucidating the exact details of such influences. A paper published in the Annual Review of Ecology and Systematics in 2000 by MacMahon et. al. reviewed available studies concerning harvester ants and explored our current understanding of the influences that harvester ants (particularly those in the genus Pogonomyrmex) can potentially have on community structure and ecosystem functions. Following are some of the direct influences the authors listed:

  • Removal and consumption of seeds and other materials – The relative abundance of plant species can be affected by the selective removal of seeds. Harvester ants also collect leaves, twigs, pollen, flowers, vertebrate feces, and arthropod body parts.
  • Storage and rejection of seeds – Collected seeds can be dropped during transport, rejected after arriving at the nest, or abandoned in nest granaries. All result in the transport of seeds away from the parent plant and dispersal beyond the plant’s primary dispersal mechanisms.
  • Construction and maintenance of nests – All vegetation and debris is removed from the area immediately surrounding the nest including mature and emerging plants. This area is kept clear for the duration of the life of the colony and, in some cases, can be quite extensive.

Harvester ants can also influence soil properties and soil food webs within and in the vicinity of their nests. They bring large amounts of organic matter down into the soil and redistribute vast amounts of soil particles. Their actions also influence the amount of moisture in the soil surrounding their nests.

This is a mere distillation of the influences that harvester ants might have; see the paper by MacMahon et al. to learn more.

In an effort to better understand how the seed predation and seed dispersal behaviors of harvester ants might influence plant population dynamics, a research team in Spain used data obtained from field research to build a computer model that would predict changes over time. The study site was described as “open and heterogeneous shrubland” and the vegetation was stated to be in “a very early stage in the secondary succession” after being subject to “recurring fires.” The harvester ant colonies involved in the study consisted of three species in the genus Messor. The plant species selected for the study were three native shrubs whose seeds were known to be collected by the harvester ants. Each plant species differed slightly in the amount and size of seeds it produced and in its primary seed dispersal mechanism, which is important because the researchers hypothesized that “the effect of seed predation and seed dispersal may depend on plant attributes.”

Messor bouvieri (photo credit: www.eol.org)

Messor bouvieri (photo credit: www.eol.org)

Data obtained from simulated scenarios and field observations appeared to support this hypothesis; each shrub species interacted differently with the harvester ants. Coronilla minima benefited from “accidental” seed dispersal. Comparatively, it produces a high amount of large seeds, which are primarily dispersed by gravity. Despite predation, ant-mediated dispersal was an advantage. Dorycnium pentaphyllum produced the highest amount of seeds among the three shrub species; however, seed predation was found to have negative effects on its population dynamics. Its primary seed dispersal mechanism involves ballistics (the mechanical ejection of its seeds), so ant-mediated dispersal may not offer an advantage. Finally, Fumana ericoides, despite its limited primary seed dispersal and its comparatively low production of seeds was not affected by the actions of the harvester ants. The authors concluded that “some unknown factor is driving the population dynamics of this species, more than the action of ants.”

Studies such as this, while leaving many unanswered questions, help us understand the important role that harvester ants play in our world. Harvester ants, and ants in general, are truly among Earth’s most enthralling and influential creatures. Learn more about their complex behaviors and countless interactions with flora and fauna by checking out these three documentaries recommended by ANTfinity.