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Wednesday, 04 September 2013 15:46

Liverworts — a unique bridge in the plant world

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mtnvoicesSome years ago, when I first became interested in plant identification, I became curious about liverworts. They are one of the distinctive plant groups (like fungi, lichens, mushrooms, etc.) without advanced vascular systems.

 

The very name “liverwort” was intriguing, but I didn’t really know what one looked like. So I studied the illustrations and texts in several plant books and went out looking for liverworts in the woodlands near my house. It was the sort of low-key “adventure” that botanizers relish. We’d rather locate a new type of plant, however mundane, than encounter a dinosaur.

I was armed with the information that “wort” means plant or herb, and that the first part of their common name derives from the fact that about one-fifth of all liverworts grow in flattened lobes (thalli) that somewhat resemble the human liver. Moreover, liverworts were reported to be “particularly abundant in rocky, moist places where the light level is too low for competing flowering plants.”

I decided to restrict my hunt to those more obvious types that display a ribbon-like thallus rather than those that closely resemble moss. And it seemed as if I needed to head down the creek from my house, where there’s plenty of shade and an abundance of rock seepage slopes along the pathway.

I’d advanced perhaps 75 feet down the creek when I spotted my first liverwort stand. A little colony was growing on a small outcrop situated in perpetual shade just above the creek. I’d walked past it hundreds of times in the past without knowing that liverworts even existed.

Several weeks later, looking out my kitchen window toward the springhead behind the house, I spotted a colony of several thousand liverworts growing along a small streambed. Which all goes to prove, I suppose, that you generally have to know what you’re looking for before you’ll actually “see” it.

I have become fond of liverworts and no longer go near a seepage area or waterfall without looking for them. In liverworts, one can observe an example of the type plant that bridged fundamental evolutionary gap between aquatic algae and the land-dwelling plants millions upon millions of years ago. Like ferns and club mosses — which represent the next step up the evolutionary ladder — they live on land and reproduce by spores but must do so in damp places because they have no protective outer layer to prevent water loss. In addition, their free-swimming sperm require a film of water to reach and fertilize the egg cells.

I have learned that liverworts exist in two forms that can be readily distinguished. First, there’s the gametophyte plant (the ribbon-like thallus); and second, there are the sporophyte plants (resembling tiny umbrellas) that grow out of the thallus and contain the male and female sexual parts.

As each liverwort plant is either male or female, colonies that reproduce successfully in a sexual manner (cross-fertilization) grow closely together — often overlapping in dense, tangled mats — so that the transmission of sperm can take place via the constant moisture covering the plants. Such a colony resembles a miniature rain forest as viewed from an airplane.

To insure reproduction when there isn’t enough moisture, liverworts also reproduce asexually by little cups or nests that form on the thallus. Inside these cups, very small spherical bodies (gemmae) appear that eventually detach themselves and germinate directly into new plants. The 

And as a final reproductive backup, some species are able to divide themselves where forks develop along the thallus strands and go their separate asexual ways. Each of these detached branches may fork again and separate, without any apparent limit, ad infinitum. 

It’s easy to cull through a dense liverwort colony and locate branching divisions that are just about to divide in this manner. It’s a system whereby the youngest part of the plant body is always in the forefront, nearest the fork, while the older, dying part brings up the rear. Curiously enough, when death reaches a fork, it creates new life.

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