Liverworts' more complex relatives, the mosses, appeared about 50 million years later after developing a stem and erect stature, and anchored by root-like rhizoids at the base. But because they too lacked an efficient internal means of conducting water, they still were limited in height, even though they towered over supine liverworts, as they do today on the steep edge of our woodland road.
Liverworts, having achieved success so early in the history of life on land, remain almost unaltered from their early ancestors. Their species today number about 8,000 distributed around the world, although this probably is a small representation of when they dominated the land long ago. But we have little idea how diverse liverworts were in ancient times, for their fossils are few and far between. The record suggests their direct ancestors were among the first invaders of land and for a time were the pinnacle of plant success.
There are two basic configurations of liverwort: broad flat ones (thallus types), and leafy liverworts that bear many tiny blades resembling leaves. A thallus liverwort, the lobed Marchantia that I inspected along the road bank, is common throughout Vermont. I don't know how many different liverwort species live in the Northeast Kingdom, but a couple hundred miles to the southwest over 185 species have been recognized, three-quarters of them rare or endangered.
Leafy liverworts are more numerous than the thallus kind, are more common in the tropics and warm places with heavy rainfall, therefore they are not as familiar to us living here. I have found them growing luxuriantly in subtropical rain forests and throughout the equatorial tropics. But we have leafy liverworts here in the Northeast Kingdom in isolated tracts, and odd "scale-moss" liverworts that live on maples outside our house, so small and dark, they hardly look alive. Other varieties -- horned liverworts and aquatic liverworts that float on ponds -- are also found in North America.
The upper surface of Marchantia's flat thallus, packed with green chloroplasts, is visibly different from the underside. It is in the top surface layers that water and carbon dioxide are absorbed to carry on photosynthesis. Underneath the thallus, almost colorless, is composed of structural cells used mostly for storage, with tiny threadlike rhizoids extending into the soil to anchor the plant and absorb minerals and more water. Peel away a liverwort and these delicate rhizoids break off and are lost.
When it comes to producing more of its kind to populate the world wherever conditions permit, Marchantia is versatile and highly prolific. Looking closely at the upper side of its thallus, you'll find a few very small cups, inside of which lie tiny "packages" called gemmules. These are bits of the parent plant, waiting to be dispersed. During a rainstorm, whenever a drop falls into a gemma cup, it splashes gemmules far and wide. If they fall onto a favorable surface, each grows into a new thallus.
A mature Marchantia thallus is either male or female, each bearing sexual organs. The disc-shaped male structure rises on a little stalk at one end of a thallus lobe, and it is from under its cap that sperm cells are set free when dew collects or rain falls. These microscopic swimming cells, each propelled by two hair-like flagella, travel through the film of moisture covering plant and soil, and home in on egg cells in a neighboring female's thallus, drawn there by an increasing concentration of female hormones. Eggs are held in an umbrella-shaped organ and after fertilization develop into the next generation that does not resemble its parents in the slightest way.
The offspring are not sexual, but produce spores that are disseminated by the wind. Each spore-bearing generation remains parasitically dependent upon its sexual parent, performing its reproductive function while still attached.
At first each little spore-producing plant hanging under the female organ looks like a tiny lollipop, but as it ripens, the bulbous end peels open, revealing spores and tiny spring-like devices called elaters, which are highly sensitive to changes in humidity. On a dry day, the elaters snap their moorings and thousands of spores are released in an explosive discharge. In this fashion a stationary liverwort gains access to a wider world and distributes its kind in the breeze. Eventually falling to the ground, a germinating spore grows into the sexual generation again.
My dog may have been unaware of the Marchantia liverworts I found so fascinating on the shady bank, but his olfactory discoveries kept him engrossed and informed about another enormous world of scent I will never know.
Â© 2014 William H. Amos