Welcome to the section on Micronaut.ch which explores, analyzes and explains fascinating microscopical structures.
The pollen grains of most conifers possess two inflatable bags. They look like little parachutes and for a long time researchers have thought that they represent some kind of flying aid.
Text by science writer Atlant Bieri
Photography by Martin Oeggerli
The bags increase the volume of the pollen grain and thus produce more lift, which prolongs its flight and hence increases the chance that it meets a female flower. But this theory remains controversial. Meanwhile, researchers have observed that grains are released in dehydrated condition due to sunny weather and warm temperatures, which trigger the flowering process. However, dehydrated conifer pollen grains do not have their bags inflated at all. Instead, they remain stowed inside the concave capsule of the grain.
Nevertheless, dry pine (Pinus sylvaticus) pollen are carried away by the slightest gust and sometimes sail with the wind over hundreds of kilometres. And the bags? Maybe they are not a flying aid after all.
There is a chance that the inflatable bags serve as water wings rather than parachutes. Such a function would make perfect sense when you look at the shape of the female flowers. Each of them looks like the downward pointing tip of a fountain pen. Attached to the tip is a drop which consists of a sugary liquid. This is the runway for the grains.
As soon as one of them touches down, the surface tension of the drop makes sure it’s not blown away again. Now the grain soaks up water, which causes the inflation of the two bags within a minute. Thus, the buoyancy of the pollen grain increases instantly and it floats upwards like a child hanging beneath its two water wings. The end of its underwater journey lies directly beneath the female ovule.
The pollen grain docks with the two bags first. This is important because exactly between them is the breaking point. It’s where the pollen tube grows through the shell and then penetrates the few micrometers of tissue to the egg in order to unite with it and complete the fertilization.
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