BEN # 194

Adolf Ceska aceska at VICTORIA.TC.CA
Mon Jun 1 00:33:47 EST 1998


                                                   
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No. 194                              May 30, 1998

aceska at 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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