The whitish, stalked, bud-like structures at the base of this common blue violet (Viola sororia) are flowers, and at about half an inch long, this is as big and showy as they get.
Unlike the blue-purple flowers that bloom in spring, these flowers start to appear in summer and last into fall. They have little or no pigment, reduced or no petals, and no nectar. They don't need these things because they don't need to lure insect pollinators. They never open, so they pollinate themselves.
In botanical terms, these closed flowers are cleistogamous (kly-STOG-amus), meaning "closed marriage." In contrast, the showier flowers of spring are chasmogamous (kaz-MOG-amus), meaning "open marriage."
Each type of flower can benefit the plant. The chasmogamous ones are cross-pollinated by insects, potentially mixing genes from different parents as pollen is carried from plant to plant. The resulting seeds grow into offspring that have gene combinations different from their parents and from each other, potentially giving them new traits that improve their survival and reproduction.
That's an advantage if their environment changes over space or time. In a population, more genetic variation among individuals increases the odds that at least a few of them will have the traits needed to grow in uneven or changing conditions.
The downside is that chasmogamous flowers are expensive. The plants spend much of their stored energy making pigments for petals, sugars for nectar, and longer stalks to lift the flowers to leaf height or above, where they will attract pollinators. If something happens to the flowers -- if they're eaten by an herbivore or if pollinators don't visit, for example -- that energy is wasted. That also puts the next generation at risk. If there are no seeds, there are no offspring.
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| The chasmogamous (open) flowers of common blue violet attract insect pollinators. The blue-purple pigments, darker nectar guides on the lower petal, and hairs on the lateral petals take much of the plant's stored energy to produce. |
Cleistogamous flowers provide a back-up, among other benefits. In terms of energy, they're much less expensive to make, and they don't rely on pollinators to make seeds. That's an advantage if the chasmogamous flowers are missing or aren't pollinated, because the plants have another way to produce seeds. It's a second chance.
Because they're self-pollinated, cleistogamous flowers produce seeds and offspring that are genetically identical to the parent and to each other. That's beneficial if environmental conditions are favorable and stable. If the parent is genetically well-adapted to the conditions, then the offspring will be, too, because those genes are preserved by self-pollination.
Self-pollination can also help get rid of versions of genes, called alleles (ah-LEELS), that reduce fitness, i.e., successful growth and reproduction. Cross-pollination can mask these harmful alleles by contributing healthier ones from other plants, thereby blunting any deleterious effects. Self-pollination, though, increases the odds that the effects of the alleles, now not partnered with more favorable ones, will show up in a plant's anatomy or physiology. That's bad for individuals that die or are unable to reproduce as a result, but the loss of those individuals can eliminate the responsible alleles from a population.
Genetic uniformity can be a disadvantage, too, especially if conditions vary across a habitat or if they change over time.. A new environment may require adaptations the population doesn't have, because the plants are genetically identical. For example, if conditions are warmer and drier but the offspring come from a parent adapted to cooler, wetter conditions, they may not reproduce or even survive.
Another potential disadvantage of cleistogamy is inbreeding depression, the loss of fitness that can result from maladaptive alleles that aren't "weeded out" by self-pollination, as described above. Also, although cleistogamous flowers usually produce more seeds than chasmogamous ones, the seeds tend to be dispersed closer to the parent plant, which can increase competition among siblings (1).
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If you find cleistogamous flowers in violets or other plants, you'll be looking at something really old. Cleistogamy developed about 100 million years ago in some of the first flowering plants and has developed in many species since then. According to one estimate, worldwide there are at least 693 species of cleistogamous plants in 50 families (1). That was in 2007; more cleistogamous plants might have been discovered or confirmed since then.
Although cleistogamy is present in a minority of plants, its long persistence among many species suggests that it has improved plant reproduction and survival. In at least 50 families, these plants converged independently on the same solution to the challenges of floral reproduction. That's profound, even amazing -- in a colorless, nectarless, inconspicuous kind of way.
Cited Reference
1) Theresa M. Culley and Matthew R. Klooster. The Cleistogamous BreedingSystem: A Review of its Frequency, Evolution and Ecology in Angiosperms. The Botanical Review 73 (1): 1-30. 2007.
Additional References
Anne L. Sternberger and others. Environmental impact on the temporal production of chasmogamous and cleistogamous flowers in the mixed breeding system of Viola pubescens. PLoS One 15 (3). 2020.
M.W. Austin, P.O. Cole, K.M. Olsen, and A.B. Smith. Climate change is associated with increased allocation to potential outcrossing in a common mixed mating species. Am J Bot.109(7): pp.1085-1096. 2022.
Theresa M. Culley. Reproductive Biology and Delayed Selfing in Viola pubescens (Violaceae), an understory herb with chasmogamous and cleistogamous flowers.International Journal of Plant Sciences 163 (1): pp. 113-122. 2002. [Available to view with a free JSTOR account.]
