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BEN # 194
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No. 194 May 30, 1998
aceska@victoria.tc.ca Victoria, B.C.
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Dr. A. Ceska, P.O.Box 8546, Victoria, B.C. Canada V8W 3S2
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THE EXPLOSIVE POLLINATION MECHANISM IN CORNUS CANADENSIS L.
From: Ted Mosquin, Box 279, Lanark, Ontario KOG I K0
Abridged from: The Canadian Field-Naturalist, Volume 99, Number
I, Jan.-Mar. 1985
[Since I read Dr. Mosquin's article published in The Canadian
Field-Naturalist, I have been poking bunchberry flowers at every
opportunity. I asked Dr. Mosquin to adapt his article for BEN. I
hope that you will enjoy playing with bunchberry flowers as much
as I do. - AC]
This is the time of year to go out and take a look at the floral
pollination method of Cornus canadensis. All parts of the flower
are synchronized to explode in a split second to affect pollina-
tion. The only other species in the genus, C. suecica (of
Eurasia) possess an identical mechanism. The basic elements of
the mechanism are: a sensitive antenna-like structure projecting
from one petal of the unopened flower bud, reflexive petals (on
a tensile 'spring'), and stamen filaments also possessing "elbow
springs" which act to catapult the pollen in the anthers upwards
toward the top of the flower. In comparison to all other 'rapid
movement" pollination mechanisms in the Angiosperms, this ex-
plosive mechanism is singularly unique.
Explosive or other rapid-movement floral mechanisms related to
pollination are rare in the plant kingdom. In known examples
where rapid movements do occur, it is the anthers or the stamens
which move more or less alone or in consort with restraining
petals. Examples described in the literature include the "ex-
plosive anthers" of Pilea microphylla Liebm. ( Taylor 1942, p.
608), often referred to as the "artillery plant" and Urtica
(H.G. Baker, pers com.) both members of the Urticaceae. Another
example occurs in Kalmia angustifolia L. (Marie Victorin 1942,
p. 466) a member of the Ericaceae where the anthers are par-
tially embedded in the petals and are simply released at
maturity catapulting their pollen in the direction of the stigma
(and at any pollinating insect) but with the petals playing a
stationary role. The examples of Medicago (alfalfa) and
Sarothamnus (Scotch Broom) both in the Leguminosae have been
widely reported in the literature (e.g. Meeuse 1961). In the
genus Lopezia (Onagraceae) and in the genus Hyptis (Labiatae)
the stamen, when touched, snaps upward and deposits pollen on
the underside of an insect visiting the flower (P. Raven, pers.
com). Some other examples brought to my attention by H. G. Baker
(pers com) are Stylidium (Stylidiaceae) Mucuna and Ilex
(Leguminosae), Odontonema (Acanthaceae) and Ravenala (Musaceae).
A number of less rapid floral movements related to pollination
are described by Meeuse (1961).
I first learned about the pollination mechanism in C. canadensis
in 1968 while conducting observations and experiments on the
reproductive biology of native plants in Banff National Park,
Alberta. It was not until 10 years later that I was to find out
that mine was not the first observation or recording of this
explosive mechanism. The first and, to my knowledge, the only
previous reference is contained in a one-line note by Marie-
Victorin (1942) in Flore Laurentienne. In his description of
Cornus canadensis L. he noted that "Les vrais petales et les
etamines vent elastiques et reagissent lorsque un insecte les
touche." He did not record any observations on an "antenna" nor
did he comment on the presence of a similar mechanism in Cornus
suecica L. although the latter species was also included in his
flora.
The inflorescence of Cornus canadensis is comprised of four
white showy involucral bracts surrounding a few to several dozen
small, relatively inconspicuous flowers. The true petals are a
very light greenish yellow to nearly white in colour. The fila-
ments and anthers are also very light coloured. However, the
ovary, style and stigma are a very dark purple and provide
striking miniature contrast points in the inflorescence. In the
Banff area the number of flowers in each inflorescence varied
from 8 to 15.
The pollination mechanism of C. canadensis is so intrinsically
interesting as well as unique in the plant kingdom that I feel
it would be worthwhile to record here a brief description of how
the discovery was made. This account is based on plants growing
near Altrude Creek about one mile south of Mt. Eisenhower Junc-
tion in Banff National Park.
Lying stomach down on the forest floor looking through a micro-
scope, I began to examine the plants searching for a series of
flowers in various stages of maturation. Normally, it is rela-
tively easy to assemble such a sequence ranging from young buds
to flowers that are very old and in a state of senescence. But
each flower of C. canadensis was either still in the bud stage
or completely open with the petals very strongly reflexed out-
wardly or downward. In all open flowers the anthers extended
upward, well above the tip of the stigma, and were empty of
pollen. While the absence of flowers in the process of opening
was puzzling, I attributed it at first to local environmental
factors and continued to examine more inflorescences. The ab-
sence of pollen in the open flowers also seemed unusual but as
the area was frequented by many species of pollen-feeding flies
(Syrphidae), I speculated that perhaps the pollen had been
collected by these colourful flower visitors.
Then with dissecting needles I began opening one of the buds,
only to discover that it seemed to transform itself in a frac-
tion of a second into a fully open flower. I turned to a second
bud, opened it and found four normal-looking, fully developed,
undehisced anthers. I tried a third and was again surprised by
what appeared to be a tiny explosion and what seemed like a
small amount of pollen flying in all directions. I realized then
that I might be looking at a unique phenomenon-perhaps never
before witnessed by humans and perhaps undescribed. It was then
that I began to pay more attention to another unusual charac-
teristic of each flower. On the abaxial side and near the tip of
one of the four petals of each unopened flower and projecting
upward was a miniature "antenna" just over one mm long. It did
not take long to establish that even the slightest touch of the
dissecting needle to the antenna of any "ready to pop" flower
would trigger the explosive mechanism; the petals would reflex,
the anthers would spring out simultaneously like four tiny
catapults and shoot their entire pollen loads into the air above
the inflorescence.
The mode of attachment of the anther to the filament, its posi-
tion, and the timing of its development within the bud cast some
light on the morphological basis for the popping action. In the
young bud, that is, one that is not mature enough to be tripped
by mechanical means, the four anthers are undehisced. As the bud
matures, anthers dehisce fully while still in the bud (but do
not release their pollen). Once dehiscence is complete the pop
mechanism seems to become activated and the slightest touch to
the antenna will cause the flower to burst open.
The attachment of the anther to the filament is basal but in the
enclosed bud the anther itself is pointed downward so that in
younger buds the attachment of the filament appears to be ter-
minal. This means that the abaxial (outside) surface of each
anther is appressed tightly against the upper part of the pis-
til.
As the bud and the pollen mature, slits are formed on the
abaxial surfaces of the anthers. At this stage both the reflexed
tip of the filament as well as the petals come under a powerful
tension which is released only when the pop mechanism is trig-
gered. During the "popping" the arched tips of the filaments act
as elbow springs and the four anthers snap upward. The "popping"
of the flowers occurs so rapidly as to be scarcely perceived by
the naked eye even through the microscope. After popping, the
stamens assume a more or less vertical position and appear to be
empty of pollen. Pollen grains are light yellow in colour and
slightly sticky; they are too large and heavy to be carried away
by the wind.
Aside from the popping mechanism, which seems to be a device
favouring cross pollination, little is known about the breeding
system of plants of this genus. When a flower pops, some pollen
is deposited on its own stigma so automatic self-pollination is
possible. It would be interesting to find out whether an incom-
patibility system is associated with the popping mechanism.
While at Banff, I failed to record a single insect visitor on
flowers of this species. However, Sadlier and Sadlier (1977, p.
100) published a photograph showing a wild species of the leaf-
cutter bee genus, Megachile, visiting an inflorescence in which
about two-thirds of the flowers had already popped. The leafcut-
ter bees are known to be major collectors of pollen (Hobbs and
Lilly 1954; Krombein 1967; Rank 1982) which they use in quantity
to provision their nest cells. My observations of Cornus flowers
showed a complete lack of nectar and it is likely that various
Megachile species, which occur throughout the North American
range of Cornus (Ivanochko 1980) are the principal pollinators.
Pollen eating flies (Syrphidae) may also be effective pol-
linators. However, in view of the sensitivity of the antenna of
any "ready to pop" flower, it would appear that even very small
insects such as woodland midges could act as pollinators should
they happen by chance to fly from one inflorescence to another.
In view of the seeming force of the popping mechanism and the
presence of the needle-like antenna, it would be surprising if
the fully mature buds did not pose some threat to life and limb
of the smaller and more fragile of the woodland insects.
The colour of the involucral bracts is creamy white and it is
known that the bracts strongly absorb ultraviolet light (UV
photo in original article). This means that the higher groups of
pollinating insects such as the bees would be able to distin-
guish the bracts from the adjacent and background objects quite
clearly ( Mazokhin-Porshnyakov 1969). Thus it is not the flowers
but rather the involucral bracts which make it possible for bees
to effect cross pollination.
The conclusion that C. suecica has an explosive mechanism very
similar to C. canadensis is based upon examination of herbarium
specimens at Agriculture Canada (DAO) and the National Museum of
Natural Sciences (CAN). The flowers of these two species are
essentially identical except that the floral parts of C.
suecica, including pistil, stamens, petals and the tiny sepals
are all deep purple in colour.
An understanding of the reproductive biology of a taxonomic
group will strengthen the foundation upon which taxonomic judge-
ments are made (Ornduff 1969). Cornus canadensis is usually
included with the genus Cornus (Marie-Victorin 1942; Harrington
1954; Scoggan 1957; Moss 1959; Munz and Keck 1968), a north-
temperate genus of shrubs and trees with some 45 species
(Lawrence 1951). Some modern European floras recognize
C.canadensis and C. suecica as a distinct genus:
Chamaepericlymenum (Shiskin 1951; Clapham et al. 1962) although
all authors with the exception of Marie-Victorin (1942) were
apparently unaware of the unique nature of the floral mechanism.
The reproductive characteristics described herein provide strong
added argument, I feel, for separate generic status for these
two species.
[In the Pacific Northwest we have yet another species, Cornus
unalaschkensis Ledeb. that originated as an allotetraploid from
C. canadensis L. x C. suecica L. - cf. Bain & Denford, Bot. Not.
132(1979):121-129. - AC]
To make certain that any species in the genus Cornus did not
contain the antenna feature so characteristic of the pop flowers
of these species of Cornus, I examined petals of flowering
specimens of all species of Cornus in the extensive herbarium
collections at Agriculture Canada (DAO) and the National Museum
of Natural Sciences (CAN) at Ottawa; all had petals with smooth
abaxial surfaces; petals lacked any semblance of pollination
structures described here.
The widely used but mundane common name of this species is
"Bunchberry." A much more interesting, meaningful and dynamic
name would be "Pop Flower."
Literature Cited
Clapham, A. R., T. G. Tutin, and E. F. Warburg. 1962. Flora of
the British Isles. Cambridge University Press. 1269 p.
Harrington, H. D. 1954. Manual of the Plants of Colorado. Sage
Books, Denver. 666 p.
Hobbs, G. A., and C. E. Lilly. 1954. Ecology of Species of
Megachile Latreille in the Mixed Prairie Region of Southern
Alberta with Special Reference to Pollination of Alfalfa.
Ecology 35(4): 453-462.
Ivanochko, M. 1980. Taxonomy, Biology and Alfalfa Pollinating
Potential of Canadian Leaf-Cutter Bees-Genus Megachile
Latreille(Hymenoptera: Megachilidae). M.Sc. thesis, Macdonald
College Library, McGill University, Montreal. 378 p.
Krombein, Karl V. 1967. Trap-Nesting Wasps and Bees: Life His-
tories, Nests and Associates. Smithsonian Press, Washington,
D.C. 570 p.
Lawrence, A. G. 1951. Taxonomy of Vascular Plants. The Macmillan
Co., New York. 823 p.
Marie-Victorin, Frere. 1942. Flore Laurentienne. Les Freres des
ecoles Chretiennes, Montreal. 916 p.
Mazokbin-Porshnykov, G. A. 1969. Insect Vision. Plenum Press,
New York [Translated from Russian by R. L. Masironi and T. H.
Goldsmith]. 306 p.
Meeuse, B. J. D. 1961. The Story of Pollination. The Ronald
Press Co., New York. 243 p.
Moss, E. H. 1959. Flora of Alberta. University of Toronto Press.
546 p.
Munz, P. A., and D. D. Keck. 1968. A California Flora. Univer-
sity of California Press, Berkeley and Los Angeles. 1681 p.
Ornduff, R. 1969. Reproductive Biology in Relation to Sys-
tematics. Taxon 18: 121-133.
Rank, G. H. 1982. First International Symposium on Alfalfa
Leafcutting Bee Management. University of Saskatchewan Press,
Saskatoon. 281 p.
Sadlier, Ruth, and Paul Sadlier. 1977. Short Walks along the
Maine Coast. The Pequot Press, Chester, Connecticutt. 131 p.
Scoggan, H. J. 1957. Flora of Manitoba. National Museum of
Canada, Ottawa, Bulletin 140. 619 pp.
Taylor, N. 1942. The Practical Encyclopedia of Gardening. Garden
City Publishing Co., New York. 888 p.
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