Cornus canadensis L.
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| Bunchberry, Cornus canadensis, flowering in northern Minnesota in mid-June 2022. |
Also called Canada dogwood or creeping dogwood, bunchberry
is a patch-forming, herbaceous plant of cool, moist forests. Although it’s
related to red osier dogwood (C. sericea), gray dogwood (C. racemosa)
and similar shrubs, this plant has no aboveground woody growth. Mature plants are
just 3-6 inches tall, their short stems tipped by four to six, arc-veined
leaves that are so closely spaced they appear whorled.
In late spring or early summer, mature plants produce a
cluster of 12-40 small flowers surrounded by four white bracts. The petals of
the flowers are just 1-2 millimeters long (1) and fused along their edges until
they open.
The stamens of the flowers grow quickly, faster than the
petals. As they mature, their anthers, the pollen-producing tips of the stamens,
are trapped inside the closed flowers, but their lengthening filaments bend
outward between the petals. Eventually, a trigger – a visiting bumblebee, for
example, or the building pressure within the flower– causes the flowers to open explosively. As the
petals flip back, the stamens spring outward, and pollen is catapulted into the
air (2,3). The grains can be lofted as high as 2.5 centimeters (25 millimeters)
above the flower, ten times the height of the flower itself (3).
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| A single bunchberry flower opens explosively -- in about half a millisecond (3). When the petals are flung back, stamens are released and catapult their pollen. Illustration based on photographs in Whitaker, et al. (2). |
[Watch a video of an exploding flower here.]
If they’re launched at high enough speed, some of the pollen
may catch in the hairs of flying insects, which then carry it to other
plants. Other pollen rides the wind. Unlike plants that are pollinated only by
insects, bunchberry pollen grains are smooth instead of sticky, and so more easily carried by a breeze (2).
A dual system of pollination is an advantage for bunchberry.
These low-growing plants are self-incompatible, so they need pollen from other
plants to form seeds. If insect pollination isn’t successful, wind pollination might
be, but for the latter to work, pollen must be launched high enough to be wafted
over a patch of the plants.
If either method of pollination succeeds, the plants will
produce bunches of red drupes, fruits with single, stony seeds. The fruits look
like berries, inspiring the name bunchberry.
Where to find bunchberry
Bunchberry typically grows in cool, moist broadleaf,
coniferous or mixed forests. In North America, its range is primarily the
northern tier of states, all of Canada, and Greenland (4). This circumboreal
plant is also found at northern latitudes in Asia.
More information
For photos and more information about bunchberry, see the Minnesota
Wildflowers page for this species.
References
(1) Flora
of North America, efloras.org. Accessed online on June 27, 2022. Formal
citation:
eFloras (2008). Published on the
Internet http://www.efloras.org [accessed
27 June 2022]. Missouri Botanical Garden, St. Louis, MO & Harvard
University Herbaria, Cambridge, MA.
(2) Whitaker, D., Webster, L., and Edwards, J. (2007). The
biomechanics of Cornus canadensis stamens are ideal for catapulting
pollen vertically. Functional Ecology 21. 219-225. DOI:10.1111/j.1365-2435.2007.01249.x
(3) Edwards, J., Whitaker, D.,
Klionsky, S. et al. 2005. A record-breaking pollen
catapult. Nature 435, 164 (2005). https://doi.org/10.1038/435164a
(4) USDA,
NRCS. 2022. The PLANTS Database (http://plants.usda.gov, 06/27/2022). National
Plant Data Team, Greensboro, NC USA.
