Atlant’s Column: Sperm in Concrete

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Welcome to the section on Micronaut.ch which explores, analyzes and explains fascinating microscopical structures.

Nothing is more important for flowering plants than their sperm. They are the carriers of the genetic makeup and its protection is essential in the struggle for life. In order to guard their sperm, the plants pack them into tiny containers, which are generally known as ‘pollen’.

Text by science writer Atlant Bieri
Photography by Martin Oeggerli

 This image shows the pollen of the Snowdrop (Galanthus nivalis). It looks fragile, but appearances are deceiving. Pollen consists of one of the most consistent and enduring materials that nature has ever produced. This miracle substance is called sporopollenin. It is a building material that is largely resistant to acid and heat – a kind of polyester-concrete-carbon-fiber mixture on pure vegetable base. To date, its exact chemical structure is a mystery to researchers, because it successfully resists all analysis procedures. The material is so tough that any pollen grain only needs a very thin coat of it. One that is a hundred times thinner than a human hair. Because of this the plants rely not quite on its protective effect.

 

Ruggedly Designed Featherweights

The world is full of dangers and insidious enemies that may find a way to destroy even the strongest material. For this reason, each pollen grain is provided with a coating of tiny fat and protein molecules. Like a sunscreen, it prevents the deadly UV rays from roasting the genetic material of the sperm. The coat also acts as an antibiotic against fungi and bacteria, giving them no chance to ever get to the goodies inside the container. Although the plants protect their sperm with natural concrete, the entire structure is still light enough that it can be carried away by the slightest gust. And even if tens of thousands of pollen grains are packed to the back legs of a bee, their combined weight will not affect the insect in its flight.

All these advantages are offset by a big disadvantage. Once a pollen grain lands on a stamp of the female flower, some kind of penis grows out of it. This is the pollen tube. It makes its way through the tissue of the pistil to the eggs that lie somewhere deep within the female flower. Through the pollen tube, the sperms finally wander out of the pollen grain and fertilize the eggs. But how can the tube ever break through the walls of its concrete bunker? It seems almost impossible. Only thanks to a trick, it still works reliably. In each individual snowdrop pollen grain a long gap (‘sulcate aperture’) is embedded. It looks as if a musketeer slit them all open with a targeted blow of his sword. This cut goes right through the outer shell, exposing the softer interior. This is where the pollen tube grows out. It is the weakest link in the high security tract of plant sperm.

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Continue reading Atlant’s Column: The Mechanical Muscle, February 2012, or go back to the archive