Some lovely liverworts

 This is probably the best time of year to look for liverworts, when they are actively producing new shoots and before taller vegetation grows up around them and often hides them in spring. Here are a few that I've found recently.

Great scented liverwort aka snakewort Conocephalum conicum, growing on a wet, shady retaining wall. This species has a powerful, agreeable scent when you crush it, which to me smells like Duraglit metal polish with citrus undertones. Prominent air pores and the polygonal pattern on the surface, give it a scaly, reptilian appearance.

Pellia endiviifolia, Endive Pellia, a thalloid liverwort whose new growth in autumn and winter consists of irregular, frilly extensions from the end of each flat thallus lobe. I also found this one on a permanently wet, shady retaining wall.

Plagiochila asplenoides, Greater Featherwort, a leafy liverwort that I found growing on a wet, shady bank in open deciduous woodland in Teesdale. One of the commonest leafy liverworts in woodland.

Plagiochila porelloides, Lesser Featherwort. A smaller, more compact Plagiochila species with darker green leaves. I found this one growing in crevices in a wet, shady retaining wall. 

Metzgeria falcata, Forked veilwort, a small thalloid liverwort that I found growing on the bark of an old beech tree, at the point where rain trickles down the trunk. The thin, flat thallus forks near the tip. 

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.

Spring quite literally springing....

It's easy to overlook liverworts. These simple plants were the first to colonise the land and have been around for over 500 million years so in terms of durability, having survived five major mass extinction that wiped out many other life forms, they are unequalled in the plant kingdom. For most of the year they are pretty dull - just a mass of ground-hugging lobed green thalli confined to permanently damp places like the banks of ditches.  But for a few weeks in spring they do something rather extraordinary.


In late winter the surface of a liverwort thallus is a hot-bed of sexual activity. Embedded in the thallus are microscopic pits that release thousands of tiny male cells that swim across wet surfaces using their whirling flagellae. They're heading towards microscopic female structures called archegonia that contain an egg cell and are located under the edge of the thallus. Once fertilisation is achieved a spore capsule forms, like a small dark bead on the surface of the thallus, and when it's ripe the spore capsule stalk begins to elongate - very rapidly.


The images here are of the thalloid liverwort Pellia epiphylla and in the top image you can see a couple of spore capsules with their stalks just beginning to elongate.

After a few hours the stalk (seta) is clearly visible and from this point it elongates very rapidly ..

... so after about a day the whole structure is a couple of inches tall and resembles a match with a glassy stalk.

Then the capsule wall splits open into four segments and the contents literally explode, when elongated, compressed cells called elaters act like springs and hurl the spores into the airstream. You can see the process in more detail and in action by clicking here..... or if you collect some Pellia from the banks of a ditch (woodlands are a good place to look for it) you can watch the whole sequence for yourself.


Simple, primitive but very effective, it has served liverworts well for half a billion years.


There's a nice time lapse sequence of elongating spore capsules of another genus  here.