Darlingtonia californica - the Cobra Lily
TRAP TYPE: Pitfall Trap
One species, Darlingtonia californica
Torr. (1853), occupying boggy habitats in the northwest United States
of America (southern Oregon, northern California).
Darlintonia californica grows along stream sides and in bogs
in northern California and adjacent parts of Oregon as shown in Figures
2 & 3. Although the distribution of Darlingtonia is somewhat
similar to the distribution of serpentine
rocks in these areas, they aren't identical at all. Darlingtonia
doesn't appear restricted to the serpentine sites, but it does appear
to grow well in areas rich in heavy metals.
The leaf of Darlingtonia is unique: it is tubular, and contains
a fluid that digests trapped insects. The upper part of the leaf, shown
in Figure 1, is modified for the trapping function. The end of the leaf
is hooded. There is a circular opening through which insects can fly
into the hood. If you look closely you can see the opening in Figures
1, 4 and 5. So why would they do so? One reason: on the two flag-like
structures, there are glands that secrete sugary nectar, such as flies
and other insects like to feed on. There are more nectar glands inside
the hood, so an insect tends to progress right into the hood.
In a few flat boggy areas, one can see thousands of Darlingtonia
plants. They prefer sunshine and don't grow well in shade. If you want
to visit a large colony of Darlingtonia plants, there is one
open to the public near Florence, Oregon, on the coast highway.
In Figures 1, 4 & 5 we can see the hood of a leaf with its flaglike
appendages. Again, notice the circular opening through which insects
enter the hood. What function do you think the flaglike appendages might
Leaves of Darlingtonia often turn reddish-purple. This is
a color that tends to attract flies and may aid in leading them to the
leaves. Notice also that the hood has translucent areas in it, especially
near the top of the hood (Figure 5). These have a clever purpose.......
In Figure 6 we see the hood the way an entering insect sees it, and
a second reason for why an insect would enter is evident. The translucent
windows in the leaf tissue look skyward, so that the insect tends to
fly upward thinking it is going outside of the hood.....
In Figure 7 we see the windows from the outside of the hood. Note the
many areas that have no chlorophyll or other pigments in the cells,
they are remarkably clear. How and why do you think a plant might evolve
in this manner?
So why doesn't an insect just buzz around in the hood until it finds
its way out through the circular opening? It is, of course, fooled by
the windows. To make matters even more difficult for those who enter,
the inside of the hood has a waxy surface, so that an insect landing
on that surface can't get a grip and tends to fall into the tubular
part of the leaf. Once in the tubular part of the leaf, shown here in
the sectional view of Figure 8, it's in trouble. Sharp downwardly-pointing
hairs, which here merely look like glistening areas, prevent the insect
from climbing back up the tube to the hood.
...and so the insect falls into a pool of liquid at the bottom of the
tube and is drowned. The pool of liquid doesn't show here because in
sectioning the leaf, the liquid drained out. But in Figure 9 you can
see the bodies of dead insects that have been trapped by this particular
leaf. The soft parts of the insects are digested and turned into nitrogen
compounds that are used by the plant. Darlingtonia, like other carnivorous
plants, grow in water that is poor in nitrogen, and the soft parts of
insects are rich in nitrogen and can supply that nutritional need to
the plant. The hard surfaces of insects cannot be digested and remain
in the leaf.
The flowers of Darlingtonia point downwards. This may keep
rain out of the flowers; wet flowers usually don't get pollinated. The
sepals are large and yellow-green. The petals seem almost hidden--they're
dark purple as shown in Figures 10 & 11.
By removing some sepals, we can see the unusual nature of the petals.
There are some pale translucent areas on the petals--these transmit
some light to the inside of the flower, and perhaps a pollinating insect
can find its way into the flower better. Also notice there are some
spaces between the petals, elliptical openings through which an insect
could enter....but what kind of insect?
Actually, nobody knows what pollinates Darlingtonia flowers!
One worker who watched the flowers didn't see any insects visiting the
flowers. In various kinds of flowers, the shapes, colors, and patterns
are usually closely related to the pollinators.
If we remove a sepal and a petal, we can see the inside of the flower
as an insect might see it. Notice one of the openings between two petals
through which in insect might enter--and the translucent spots in the
petals show up well in this daylight photograph. Several things about
the Darlingtonia flower suggests that a beetle might visit it--but remember,
nobody knows yet.
If we look at flowering plants of the world, those pollinated by beetles
are frequently dark purple and green. Bees don't pay attention to those
colors. Some flowers that are pollinated by beetles have traplike shapes--and
the flowers of Darlingtonia fit that description. The reason why beetle-pollinated
flowers have a traplike shape is that beetles don't fly in and out of
flowers very efficiently--beetles tend to stay in flowers and feed on
flower parts. By bumbling around inside a flower, beetles can pick up
a lot of pollen or deposit it on the pollen-collecting sturfaces, the
stigmas (the forklike structures in this picture). The longer a beetle
stays inside a flower, the more likely it is to pollinate the flower.
Beetle-pollinated flowers don't have nectar, whereas flowers pollinated
by bees, flies, and birds do have nectar. Darlingtonia flowers have
no apparent nectar, so that would fit the beetle idea. Beetles like
to eat pollen--and as you can see, Darlingtonia flowers produce lots
of pollen, which the flower is shedding here from its pale yellow stamens,
shown above. If beetles eat the pollen, wouldn't that be bad for pollination?
No, it's a compromise. Beetle-pollinated flowers produce plenty of pollen,
and beetles aren't very efficient feeders, so there always seems to
be plenty of pollen left over for a beetle to carry around on its surfaces,
transferring that pollen to another flower.
After pollination occurs, the fruit expands and turns brown when it
matures. Some splits occur in the sides of the fruits, and some of the
seeds might be dispersed through those splits.
But more likely, the seeds escape through some openings at the end
of the fruit. Notice they mouthlike openings at the end of the fruit.
These close up if the fruiit gets wet during the rain, but they open
ujp when the weather is dry and windy. This suggests that the seeds
are dispersed by wind. The fruit of Darlingtonia is like a
shaker--only a few seeds escape at a time, because the mouthlike openings
are small. This benefits the plant, because dispersing seeds over a
period of weeks means that the seeds will reach more places. Also, more
seeds are dispersed on a windy day, because more seeds sift out through
the openings. If seeds of a plant are wind-dispersed, a mechanism like
this that disperses more seeds on a windier day is better, because the
seeds will be carried farther, and Darlingtonia seeds could reach distant
places where it could grow well.
Here's what the seeds look like just before they mature. This is a
cross-section of a fruit. The seeds are still attached to the central
part of the fruit. When the seeds mature, they break away from the center
of the fruit and gradually fall out through the openings shown in the
previous picture. The places where Darlingtonia can grow--streams
and bogs--are only small areas. Small seeds carried by wind would reach
these small, distant areas better than would bigger seeds.
Darlingtonia seeds are small (Figure 19) --only about 3 mm
long. Nobody has done research on how the seeds are dispersed--a fun
project! Here's some guesses about what someone might find... The seeds
sift out of the seed pods on dry windy days, and the hairs on the seeds,
by increasing the surface area, permit the seeds to float farther. Possibly
the stiff hairs help the seeds to get picked up by wind gusts when they
fall to the ground. Streams and bogs suitable for Darlingtonia
are like tiny islands in the land, so wind dispersal of seeds would
be an ideal mechanism for getting them to these isolated areas. If the
seeds fall onto water, they may float for a while because air bubbles
get trapped among the hairs. If they are floating, the hairs may aid
the seeds in catching on debris along the side of a stream or bog, and
thus the seeds would reach a place suitable for their growth.
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