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© 2001 All Rights Reserved.

Volume 2, Number 1
Winter 2001
On line Version


Natural Pollination
of Viola Flowers
By  Theresa M. Culley, Ph.D..

Theresa M. Culley, Ph.D., a member of the board of The American Violet Society, holds a postdoctoral position at the Department of Ecology & Evolutionary Biology, University of California, Irvine. Her research interests include plant population ecology, pollination biology, conservation of rare species (especially Hawaiian endemics), population genetics, statistics, and community ecology.

Theresa's Current Violet Research


Image of Downy Yellow Violet Chasmogamous (CH) Flower
(Figure 1.)  Chasmogamous (CH) Flower Of The
Downy Yellow Violet, Viola pubescens

            Violets are most widely recognized for their beautiful, vibrantly colored flowers that appear in the early spring (Fig. 1). Known as chasmogamous flowers, these are different from the bud-like cleistogamous flowers that are produced later in the season. Chasmogamous flowers have been referred to in the literature, painted through the centuries, and even appear today on everyday items such as glassware, linen, and clothing. Despite the popular attention to these flowers, scientists have not completely understood their purpose.

             People have often assumed that chasmogamous flowers were produced to attract insects, resulting in outcrossing between two different plants. This is also suggested by the flower name itself - chasmogamous - which is Greek for "open marriage" (i.e., a union between two plants). The renown violet expert, Dr. Ezra Brainerd once suggested that in several stemless species, "the structure of the flower, in style, stamens and petals, shows a most ingenious arrangement to prevent self-pollination" (Clute, 1907). However, chasmogamous flowers in some non-violet species were recently found to self-pollinate. Because I was studying Viola at the time, I was interested in examining chasmogamous flowers to determine if selfing could also occur in some stemmed members of the genus. It was already known that selfing was not possible in flowers of V. riviniana, V. odorata, and V. canina (Knuth 1908; Beattie 1969; Banasinska and Kuta 1996).

             I studied the downy yellow violet, Viola pubescens (Fig. 2), because little was known of its floral biology even though it is so common in northeastern beech-maple forests. Over four years, I found that the chasmogamous flowers were only occasionally visited by insects. These included bee flies, skipper butterflies, bumblebees, carpenter bees, and halictid bees (Fig. 3). To determine if chasmogamous flowers of V. pubescens were able to self-pollinate, I covered plants in the forest with mesh cages (to deter pollinators). The majority (59%) of flowers on these plants still produced seeds, indicating that the chasmogamous flowers were able to self-pollinate. Selfing occurred at the end of an unvisited flower's life as the stigma slowly bent down and touched pollen grains that had fallen out of the anthers and were resting on the bottom petal. Known as delayed selfing (because it only occurs after opportunities for insect visitation have passed), this mechanism was previously unknown in V. pubescens, but has been found recently in another stemmed violet, V. canadensis (Culley, unpublished data). There also is a brief reference to this type of selfing in V. arvensis in a 1902 book by Anton Kerner von Marilaun.

             Once I realized that the chasmogamous flowers of V. pubescens were able to self-pollinate, it was important to find out how frequently this occurs in nature. With seeds I had gathered from forest plants, I used a laboratory technique to determine the rate of selfing over a two year period. I discovered that over half (60%) of the seeds produced in chasmogamous flowers the first year resulted from self-pollinations. This was contrary to the frequent assumption that the elaborate chasmogamous flowers are largely outcross-pollinated by insects. However, only 7% of the seeds originated from selfing the following year. This difference in selfing rates was probably due to fewer visits by pollinators in the first year, which resulted in delayed selfing within the unvisited flowers (that is, higher selfing rates).

Image of Downy Yellow Violet Cleistoogamous (CL) Flower
(Figure 2.)  Cleistogamous (CL) Flower Of The
Downy Yellow Violet, Viola pubescens

             So why do chasmogamous flowers of Viola pubescens retain the ability to self-pollinate, even though they are so vibrantly dressed in an attempt to lure insect visitors? This is especially puzzling considering that the flowers of many other violet species, such as those Dr. Brainerd studied, are not able to self-pollinate. The answer may lie in the fact that the insect visitors of V. pubescens are very unpredictable. In four years of observations, I only saw pollinators visiting the flowers during one season. Thus, the plants may not always be able to count on insects being available to pollinate them. Because the showy chasmogamous flowers require so much energy (nutrients, etc.) to make, it would be best for the plant to produce seeds through selfing, rather than waste the entire flower if pollinators were absent. Thus, V. pubescens and similar violet species have an advantage over other plants that require insect visitation to produce seeds.

             The next question that remains to be answered is to determine why chasmogamous flowers of other violet species cannot self-pollinate, especially given that it is such an advantage. Perhaps these plants have adequate pollinators each year, or there may be other reasons. Hopefully, this question will be answered in the next few years, as the popularity of Viola continues to spread and more people are intrigued by its fascinating floral biology

© 2001 Theresa M. Culley, Ph.D.
For The American Violet Society
All Rights Reserved


Literature Cited 
Banasinska, A., and E. Kuta. 1996. Allogamy in Viola odorata L. Acta Biologica Cracoviensia Series Botanica 38: 41-51.
Beattie, A.J. 1969. Studies in the pollination ecology of Viola. I. The pollen-content of stigmatic cavities. Watsonia 7: 142-156. 
Clute, W.N. 1907. Violet Seeds. American Botanist 13: 86-87. 
Kerner von Marilaun, A. 1902. The natural history of plants. Vol. II. Trans. by F.W. Oliver, Blackie and Son, London. 
Knuth, P. 1908. Handbook of flower pollination: based on Hermann Müller's work 'The fertilisation of flowers by insects'. Vol. II. Trans. by J.R.A. Davis, Clarendon Press, Oxford. 

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