A role for water droplets in the pollination of Platanthera aquilonis (Orchidaceae)

7 Pages·2001·2.9 MB·English

by  C J Sheviak

RIIODORA,
No. 380 2001
Vol. 103. 916, -386,
pp.
A ROLE FOR WATER DROPLETS THE POLLINATION
IN
OF PLATANTHERA AQU1LON1S (ORCHIDACEAE)
Charles Sheviak
.1.
NY
New Museum.
York Albany, 12230
Biological Survey, State
[email protected]
e-mail:
abstract. The North American Platanthera aquilonis Sheviak has long
known downward
forward and
been to autopollinatc. Pollinia rotate oul o\'
the anther sacs and contact the stigma. Previous experimental evidence has
movement move-
indicated that this effected by gravity. In addition to the
is
ment of whole pollinia, flowers are often found in which continuous streams
down
of pollen massulae emanate from anther sacs and seemingly pour onto
shown
the stigma, Experiments with cultivated plants have that water droplets
These sometimes draw
can collect in the centers oi flowers oi' this species. a
sheet massulae out of an anther sac either onto the surface the droplet
oi' oi'
As
or into interior. the droplet evaporates, the massulae are deposited as
its
continuous layer from the anther sac onto the stigma. suggested that
a is
It
dew may
nature function to produce the observed pollen streams.
in
Key
Words: Platanthera aquilonis, Platanthera hyperborea, pollination
The Sheviak
recently described Platanthera aquilonis a
is
widespread and well-known North American species has
that
been confused with the Icelandic hyperborea (L.) Lindl. (Shev-
P.
Autogamy American
North species has long
iak 1999). in the
been known, having been reported [as Habenaria hyperborea (L.)
Asa Gray
R. Br. by (1862a, 1862b). Shortly after the flower
|
weak
sometimes
opens, or while the bud. the caudicle
in
still
bends and the pollinia are said to onto the stigmatic surface
fall
below (Gray 1862a, 1862b; Catling 1983; Catling and Catling
1991). Catling (1983) determined that the motion evidently
is
driven by gravity, as did not occur experimentally inverted
in
it
flowers.
My
experience with cultivated plants o[ Platanthera aquilonis
much
largely confirms these earlier findings. In plants from across
New
the range of the species, from Alaska to Illinois and York,
oi'
bending the caudicle rotates the polliiiium forward through
a
ol'
downward
and onto
a longitudinal fissure in the anther sac the
may
newly opened
stigma. This process be evident (lowers or
in
A common
may
be delayed for several days. feature ol' plants in
the however, and clearly evident the herbarium, suggests
field, in
380
—
Sheviak Platanthera aquilonis 381
2()()l|
The
that an additional process work. pollinia are very loosely
is at
organized (again as reported by Gray 1862b), with massulae
(i.e.,
primary groups of pollen tetrads) only weakly bound secondarily
into easily fragmented pollinia. Quite commonly, loose massulae
emanate from
anther sacs enclosing remnants of
the pollinia
still
How
down
and
trail onto the stigma. this condition arises has not
previously been reported; evidence suggesting a plausible caus-
agent presented
ative here.
is
AND METHODS
MATERIALS
A
number
of plants of Platanthera aquilonis from various
sites
across the range of the species have been maintained cultiva-
in
many They grown modern
tion for years. are a office building
in
under a motorized track-mounted lOOO-watt metal halide lamp.
The area has been used for over 20 years to grow plants of var-
ious species of Platanthera, Spiranthes, and other genera. Con-
maintenance normal phenotype
ditions are suitable for of expres-
many
sion, and individual plants have survived throughout
this
period. 1997 one plant of aquilonis from (McHenry
In P. Illinois
New
Co., Sheviak 5599), and 1999 two plants from York (Es-
in
sex Co., Sheviak 6229) were employed experiments. Temper-
in
atures were maintained within a few degrees of 2()°C. but hu-
midity varied widely.
1997 produced and
In the Illinois plant a large inflorescence,
flowers opened over a period exceeding one month length.
in
Despite this protracted interval, near the end of the period no
flowers had been pollinated and pollinia remained enclosed
all
within the anther sacs. In the several years of cultivation of plants
of this species, this was the case noted in which autopolli-
first
nation had not occurred. The humidity in the growing area at that
was
time uniformly quite low, so suspected that the anther sacs
I
might have been unusually rigid, thereby trapping the pollinia.
This provided an unusually clear opportunity to investigate the
possible role of humidity pollination, since autopollination was
in
not occurring by any means, and humidity could be readily ma-
nipulated.
In the initial trial in 1997, a very fine mist of water from a
pump
hand atomizer was directed the inflorescence of the
at Il-
and was enclosed
linois plant, the plant a colorless transparent
in
plastic cylinder closed at the top but open at the bottom to permit
3S2 Rhodora
[Vol. 103
The
limited air exchange yet retain humidity. walls of the cylinder
humid The chamber was
were also sprayed to create a chamber.
some
removed
for the night, and. following initial positive re-
was
sponse was evident the following morning, the misting
that
repeated. 1999 verification was attempted, and the same pro-
In
New
cedure was repeated on the two York plants.
RI-ISULTS
numerous
With misting of very small
the the Illinois plant,
first
many
drops of water formed on the segments. In cases these
floral
immediately coalesced to form a single large drop that occupied
the center the (lower, immersing the column. After 90 minutes
oi'
was
the water had evaporated from the flowers. At that time there
morning
no noticeable change the flowers, but the following
in
several pollinia were found to be protruding from the anther sacs;
one was nearly completely free of the sac. With the second mist-
ing, drops again formed in the centers o( the flowers. In several
(lowers, within 30 minutes of application, the massulae were
drawn out onto the surface o( the water droplet as pulled by
if
As
the surface tension. the water evaporated, the pollen main-
tained position. Eventually, following the complete evapora-
its
tion oi the water, the pollen remained, forming a continuous sheet
from the surface the stigma to the remains the pollinium
oi' o\'
within the anther sac.
New
The York plants were misted within a few days the
o\^
flowers' opening. At that time pollinia were already appearing
The
and a few flowers had been pollinated. flowers this time
proved to be more hydrophobic than in the preceding example,
with the result that water tended to drip the flowers rather
o\i' oi'
than be drawn into them. Nonetheless, some drops did form in
The were somewhat
the centers of flowers. results in these cases
>
New
One
Figure the dowers oi Platanthera aquilonis from York
oi'
1.
showing autopol nation effected by a central water droplet. All to same scale;
I i
= mm.
scale bar Top: Front view before treatment. Pollinia (white bodies)
1
-
are fully exposed within the open anther sacs; a pollinium {one of two); b
- stigma. Lower Oblique view showing central water droplet (note arch-
left:
-
ing reflections on surface oi droplet), c Pollinium on the left is free oi the
anther sac and riding on the surface the droplet; pollinium on the right
is oi'
—
Sheviak
200 Platanthera aquilonis 383
1
]
partially fragmented with massulae dispersing on the droplet surface. Low-
is
view The
er right: Front after evaporation of the water. massulae of the
left
pollinium are dispersed across the stigma. Most of the right pollinium remains
down
within the anther sac, but a stream of massulae extends from the sac
across the stiema.
384 Rhodora
Vol.
l()3
I
different than the preceding In most cases whole pollinia
in trial.
were drawn sometimes onto Only
drop, or surface.
into the
its
massulae stream and then sometimes occurred
rarely did out, this
shown
Examples
within drop rather than on surface. are
the
its
Figure With the evaporation of the drop, whole pollinia were
in
1.
some mouth
deposited on the stigma, or cases within the o(
in
where
the spur or even on a floral segment. In those few cases
separated massulae emanated from the anther sac, a sheet o( pol-
was The massulae
len deposited across the stigma. in these plants
more hound
appeared tightly together than in the Illinois plant,
with the result that the pollen was not deposited so evenly as in
case.
that
DISCUSSION
These mind
results bring to the cases Liparis loeselii (L.)
oi'
common
and
L. C. Rich., a associate Platanthera aquilonis,
oi'
the tropical Occeoclades maculata (Lindl.) Lindl. In these spe-
however, impact o\ raindrops has been implicated
cies, the actual
in pollination. In Liparis, the drops have been found to physically
exposed and ma-
dislodge partially pollinia (Catling 1980), in
().
impact apparently removes anther cap and permits
culata, the the
hang where undetermined
the pollinia to near the stigma, another,
Ackerman
(Gonzalez-Diaz and
agent effects actual pollination
1988).
The
very limited reported here indicate plausible origin
trials a
for the pollen streams that had previously been noted on Pla-
tanthera aquilonis flowers the held and herbarium. Addition-
in
ally, these trials account for the placement of pollinia on perianth
surfaces and within spurs, as has also sometimes been seen. In
aquilonis any wetting the inflorescence that leads to for-
P. ol'
may
mation
droplets the centers the flowers facilitate
ol' in o\^
The
pollination. coalescence very small droplets into larger
ol'
was
central ones that seen in the present experiments suggests
dew commonly
might function capacity. Rain might
that in this
some was
also cases be effective, but not simulated during
in
it
hooded and hydrophobic
study; the perianth rather sur-
this
its
common
may
faces tend deflect raindrops. Less meteorological
to
conditions, such as blowing tog or mist, might function similarly
dew, but they would not be expected to be significant except
to
common
where were phenomena.
certain areas they
in
—
2001] Sheviak Platanthera aquilonis 385
The
would
frequency of suitable droplet formation be expected
to vary with the habitat, and the relative influence of gravity- and
may
water-assisted pollination vary accordingly. Certainly the
lat-
may
ter lacking unless water present, but be important
is is in
it
dew
areas with frequent formation during Furthermore,
anthesis.
partially offsetting the positive effects of droplet formation pol-
is
len loss through deposition spurs and on perianth surfaces.
in
dew
Hence, areas where frequent during anthesis, incoherent
in
is
pollinia might represent part of a water pollination syndrome sub-
ject to selection through increased pollination and reduced loss
may
of pollinia. In this regard, be significant that the Illinois
it
was
plant, with very loosely associated massulae, from an
its
dew common
open wet where formation was
prairie likely a oc-
New
currence. contrast, the York plants were found a dense
In in
of Tsuga canadensis Carriere with Acer saccharum Mar-
forest
dew
shall on a rather dry hillside; in such a situation formation
uncommon
probably an occurrence, and
the less fragile pollinia
is
may
of these plants reflect a prevalence in this population of
gravity-assisted rotation and a rarity of water pollination.
This speculation on possible evolutionary significance of these
observations based on very small sample and obviously
a a
is
larger and geographically more diverse sample necessary
fov
is
mechanism,
verification. Further study of this novel pollination
especially natural populations occurring under diverse condi-
in
tions, might prove valuable. In addition to gravity- and water-
assisted autopollination, the flowers retain the structures neces-
sary for insect pollination, thus suggesting a third dimension that
some
might be important Platanthera aquilonis thus
in situations.
may
be a significant subject for study of the evolution of polli-
mechanisms.
nation
M.
ackno\vu-ix;mhnts. wish thank Catling review-
to for
P.
I
ing an early draft of the manuscript and for his very helpful and
insightful suggestions. Vouchers are deposited NYS.
at
LITLRATURL C1TLD
Catling, M. autogamy
1980. Rain-assisted in Liparis loeselii (L.) L. C.
P.
Rich. (Orchidaceae). Bull. Torrey Bot. Club 107: 525-529.
Autogamy
1983. in eastern Canadian Orchidaceae; a review of cur-
.
rent knowledge and some new observations. Natural iste Canad. 10: 37
1
53.
386 Rhodora
|Vol. 103
A
and V. R. Catunc. 1991. synopsis of breeding systems and pol-
North American Orchids. Lindleyana 187-210.
lination in 6:
Gonzalez-Diaz, and Ackerman. and
N. D. 1988. Pollination,
fruit set,
J.
seed production the orchid, Oeceoclades maculata. Lindleyana
in 3:
150-155.
Gray, Enumeration
A. 1862a. the plants Dr. Parry's collection the
o\' o\' in
Rocky Mountains. Amcr. 249-261.
J. Sci. Arts. Ser. 2. 34:
1862b. Fertilization of Orchids through the agency of insects. Amcr.
.
420-429.
Sci. Arts, Ser. 2. 34:
J.
Shhviak. C. 1999. The identities of Platanthera hyperborea and huro-
P.
J.
new
nensis, with the description o( a species from North America. Lin-
dleyana 193-203.
14: