|Liverworts are seldom if
ever noticed by most of us, and if we do see them usually we're not aware that we are
seeing something special. The picture at the right shows a liverwort found in the dim
light and on the moist soil of the north side of a well near my home. Each liverwort body
in the picture is about as thick as a thin pencil.
You can see that this plant is very unlike anything else we consider here. It's essentially a green ribbon growing across the ground, with each segment regularly forking into two new branches.
LIVERWORTS ARE PRIMITIVE
One reason liverworts are so curious is that in terms of the evolution of life on Earth, these plants are old. The first liverworts arose as green alga was making its transition onto land during the Devonian Era some 400,000,000 years ago -- and that's a long, long time before more advanced plants such as flowering plants, ferns and mosses appeared. In fact, liverworts are often referred to as "the simplest true plants." Here are some easy-to-see aspects of their primitive nature:
TWO MAIN KINDS OF LIVERWORTS
The 8,000 or so species of the Earth's liverworts are usually divided into two groups -- the thallose and the leafy liverworts. The ribbon-like green liverwort in the picture above is a good example of what a "thallus" is. Leafy liverworts are very tiny plants with even more miniscule scale-like lives arranged on a hair-like stem. In fact, in the picture above, the conspicuous thallose liverwort is growing over a mat of much smaller, hardly visible leafy liverworts!
When you find a thallose liverwort it's fun to look for a structure peculiar to certain liverwort species. That's the cupule in which you may be able to find gemmae. In the picture at the right the bowl-shaped items are cupules, and in each cupule you can see little bumps, like green eggs in green nests. The bumps are gemmae. Liverworts do have sexual reproduction, but some of them, such as the species shown here, also reproduce asexually (vegetatively), with no sex involved. The way they accomplish this asexual reproduction is with the cupules with their gemmae. Raindrops splash into the cupules, knocking gemmae onto the ground. If the gemmae land flat and all its other environmental needs are met, rhizoids develop on their lower surfaces and a new thallus -- a new liverwort plant -- is produced. When you go looking for cupules and gemmae, take a magnifying glass because these structures are small. The cupules in the picture are only about 1/16th of an inch wide (1.5 mm). I found this cupule-bearing liverwort in late fall, on the bank of a deeply shaded ravine near my home.
Finding liverworts during the sexual part of their life cycle is even more exciting. At certain times liverworts develop little umbrella-like structures at the edges of their thalli, called receptacles. Male liverworts produce male receptacles (technically called antheridial receptacles), and of course female liverworts produce female receptacles (archegonial receptacles). Female receptacles are deeply lobed like the ones in the picture.You might guess that when the male plant's sperm reach a female plant's eggs, a zygote is formed, and then an embryo.
The embryo then develops into a capsule filled with spores. At the right you see a much-magnified view of an archegonial receptacle from below, showing several spherical capsules filled with brown spores. As among the mosses, soon the capsules will rupture and spores will be released. If a spore happens to land where environmental conditions are just right for it, then the spore will germinate into a liverwort.
A number of thallose liverwort species are aquatic -- they grow on the water's surface like flecks of lettuce.
Liverworts are called liverworts because long ago the people who named them felt that the curious arrangement of cells on the surface of some liverworts was similar to the cell arrangement in actual livers taken from animals.
In the scheme of plant classification, mosses and liverworts, along with hornworts (a kind of plant even less known than liverworts) , are known collectively as bryophytes.
If you want to look for liverworts around your home, look in deeply shaded, moist areas, such as on he ground beneath shrubs on the north side of your house (if you live north of the Equator). Most species like cool, moist and shaded areas.
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