Pollen is a valuable commodity. The
less pollen needed, the more energy is left for other functions.
Flowers resort to several strategies to economize on pollen. Some use
a method called explosive pollination, others resort to buzz
pollination. Both methods are well illustrated by official state
flowers.
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| Mountain laurel in bloom. © Beatriz Moisset |
Explosive
Pollination of Mountain Laurels
Mountain laurel
(Kalmia latifolia),
Pennsylvania’s and Connecticut's state flower, has a singular way
to ensure that its insect visitors carry pollen to other flowers of
its species. The unopened blossoms present little knobs which
give them a funny look. Their function becomes apparent when the
flower opens. They are pockets that hold the anthers trapped. Anthers
are the part of the flower that produces male
cells, pollen.
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| Mountain laurel flower before an insect's visit. © Beatriz Moisset |
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| Flower tripped by an insect. © Beatriz Moisset |
In most flowers the anthers are free
and exposed to spread their pollen at the slightest touch of a flower
visitor. The mountain laurel has a different strategy. The pollen is
well protected against rain and wind; but, when a pollinator lands on
the flower searching for nectar, the weight acts as a trigger,
causing the taut stem of the anther to spring. The anther hits the
pollinator gently on the back and gives it a dusting of pollen.
This method allows mountain laurel
flowers to produce only a moderate amount of pollen because most of
it ends up where it is intended, on the body of a pollinator, rather
than being wasted in other ways.
Connecticut:
Mountain laurel
Pennsylvania:
Mountain laurel
Explosive
Pollination of Pea-like Flowers
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| Texas Bluebonnet. Wikicommons © Jacopo Werther |
Other
flowers encase both the pollen-producing parts and the female parts,
or stigma, into a sheath. Some members of the pea family, Fabaceae,
have flowers that resemble a butterfly, this is why this subfamily is
called Papilionoideae, papilio
meaning butterfly. The top petal is large and shaped as the sail of a
ship and is called the banner. Two side petals are called wings, and
the bottom two petals are merged into one forming a sheath that
encloses both the anthers and the stigma. This structure looks like
the keel of a boat and it is so called.
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| Structure of a papilionoid flower. Wikicommons. © David
Richfield |
When a
pollinator lands on a blossom, its weight triggers a mechanism
causing the keel to spread open and the anthers and stigma jump up
out of the enclosure. This is called tripping. The insect has to be
heavy enough to cause such reaction. Some bumble bees are pros at
this task; honey bees, on the other hand, dislike being pounced upon.
After a few times, they learn to sneak around and steal the nectar
through a small opening at the base of the keel. Only novices perform
pollination. Once they learn this trick no more pollen is transferred
to the stigmas. Nevertheless managed honey bees are widely used to
pollinate alfalfa and clover. Bee hives provide a large labor force
that makes up for this deficiency. Some bumble bees, especially the
short-tongued species, are also inclined to a little larceny.
However, they are generally considered highly competent pollinators
of alfalfa and clover.
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| Clover, another papilionoid flower. © Beatriz Moisset |
Texas’
bluebonnet (Lupinus) and
Vermont’s red clover (Trifolium pratense)
need to be tripped. Six species of lupines, all of them called
bluebonnets because of their appearance, live in Texas. All six of
them are considered the state flower. The red clover, Vermont’s
state flower, on the other hand, is not a native plant. It is an
important crop used to feed livestock.
Texas:
Bluebonnet
Vermont:
Red Clover
Buzz
Pollination or the Salt Shaker Technique
Other flowers
make the pollinators work for pollen in a different way.
Interestingly enough, honey bees never learned how to do the job I
will describe, but native bumble bees and numerous species of bees
are real pros.
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| Azalea Andrena bee pollinating azalea. Observe the pores at the tips of the anthers. © Beatriz Moisset |
In most flowers
the pollen is made available to pollinators as soon as it is ripe.
The anther splits open. We all have seen the golden dust many flowers
have. If we touch it, it sticks to our fingers. Rhododendrons and
azaleas do something else. They keep the pollen enclosed inside the
anther. It can only come out through a small opening at the tip.
Several other native flowers, like tomato and blueberry have a
similar characteristic.
In order to
extract it, the bee has to cling to the anther and give it a good
shake. It accomplishes this by vibrating its flight muscles while
keeping the wings still; it is like running the car engine in
neutral. Pollen comes in clouds and clings to the body of the
pollinator. Later, the bee proceeds to package its loot in the little
baskets of the hind legs.
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| Buzz pollination of azalea. Video. © Beatriz Moisset |
Two states have
official flowers that exemplify this method of pollination,
Washington has chosen the Coast Rhododendron (Rhododendron
macrophyllum) and West Virginia has
another species of Rhododendron (Rhododendron
maximum). Georgia’s official
wildflower, azalea (Rhododendron
prunifolium), follows the same
system of pollination; its official flower is the rose; but,
fortunately, they decided to honor a native plant in addition to a
non-native flower.
Georgia
(official wildflower): Azalea
Washington:
Coast Rhododendron
West
Virginia: Rhododendron
Other
plants select their pollinator clientele through other methods. Some
hide the nectar rather than the pollen. A different set of skills are
needed to deal with these flowers as we'll see in the next post.
References
