Pellia

Thursday's Guardian Country Diary is all about this very common thalloid liverwort Pellia epiphylla. It's hard to imagine a less charismatic plant but it is remarkable, in several ways.

It's confined to places that are permanently damp, so a shady ditch bank like this suits it well. It consists of a flattened green thallus that's anchored to the substrate by hair-like rhizoids.

Lobes of the thallus overlap and at this time of year make active growth, visible as the brighter green areas in this image.  

If you cut a thin vertical section through the thallus of the liverwort in mid-winter this is what you see (ignore those small, bright objects at the bottom of this image - they are starch grains).Those big round objects embedded in it are male antheridia, full of male sex cells called antherozoids, each of which is just a few thousanths of a millimetre long. Each is equipped with flagellae so when the antheridia burst the antherozoids can swim in the surface film of water. They are heading for .....

.... these female structures called archegonia, situated near the tip of the thallus. There are two here and you can see a dark egg cell inside one of them. The neck of each archegonium emits chemical signals that attract the swimming antherozoids. On mild, wet winter days the whole surface of the liverwort would be covered with tens of thousands of antherozoids, frantically attempting to swim to an archegonium. A few, perhaps assisted by rain splash, make it to their destination, swim down the neck of an archegonium and fertilise the egg cell.

So, in late February and early March, if you get down on your hand and knees with a magnifying glass you can see that this has been the site of very intense sexual activity. The evidence lies in those small, dark spherical structures on the surface. They are the first sign of  capsules full of spores forming, which develop from the fertilised egg.

Within a few days they become larger and darker and begin to rise on stalks - and that's a good time to bring some indoors and watch what happens next.

This picture was taken about 12 hours after the one above and the stalks have grown to about an inch long, while the capsules ...

... have become shiny black spheres.

The capsule stalk continues to grow until it's about three inches long, bending towards the light, then ...

..... the capsule explodes, in slow-motion!

It splits into four segments and a mass of green spores erupts, flicked out by hair-like cells called elaters.

After about five minutes most of the spores have been flicked out and have blown away ...

... leaving behind a mass of writhing elaters, before the stalk collapses

Here, under the microscope at low magnification, you can see the spores and their elaters.

At 400x magnification the structure of the elaters is visible. Each long cell has a spiral of thickening in its wall and when the elater is confined inside the intact capsule it is compressed like a spring. When their confinement is relieved the elaters act as biological springs, hurling out the spores.

Thalloid liverworts are the most ancient living land plants and have a fossil record dating back some 450 million years. When they first evolved those three inch-tall spore capsules would have been the tallest terrestrial vegetation on the planet. 

Today's thalloid liverworts seem to be very similar to their early ancestors. Somehow these simple plants have survived five great mass extinctions and if you measure success in terms of durability alone, then they are unequalled amongst terrestial plants.