Thursday, June 25, 2015

Thrifty flowers

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.

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.

Mountain laurel flower before an insect's visit.  © Beatriz Moisset
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


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.
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.

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.

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.
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


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