Difference Between Liverworts And Hornworts
The first plant to emerge onto dry land some 400 million years ago may have looked like a liverwort. Indeed, the oldest liverwort fossil (Hepaticites) is thought to be nearly that old.
Liverworts get their name from their simple shape, which somewhat resembles that of a flattened liver. This tiny plant has the simplest of tissues and a little waxy covering (cuticle) over its body.
Liverworts are most common in the tropics, although they occur around the world. Typically, they grow on moist, shaded stream banks, forest floors, rocks, and tree trunks. Like mosses, liverworts play an important role in breaking down rock to form soil. Today there are more than 8,000 species. There are two basic types of liverworts: thallose liverworts, which have flattened, ribbonlike bodies that form several small, rounded lobes; and leafy liverworts, which produce small, fernlike fronds covered with rows of “leaves.” Liverwort “leaves,” like those of moss, are not true leaves, as they consist only of a single layer of cells (with no true leaf tissue).
The typical life cycle of these plants is well illustrated by Marchantia, a common thallose liverwort. Like all bryophytes, it goes through an alternation of generations, with the gamete-producing generation being that which we see as the green plant. This green gametophyte arises from the germination of male and female spores. When mature, it produces male or female sex organs that grow as small stalks from its flattened body. The male sex organ, or antheridiophore, bears a disk-shaped cap, looking much like a tiny mushroom. The female archegoniophore looks more like a tiny palm tree or ragged umbrella. Rain or heavy dew enables sperm from the antheridiophore to swim to a nearby archegoniophore and fertilize the eggs there.
Each fertilized egg produces a new generation, or sporophyte. The sporophytes remain attached to the underside of the archegoniophore “palm tree” like so many tiny coconuts. The inside of the sporophyte becomes filled with spores. It also contains threadlike cells called elaters. When the sporophyte opens to release its spores, the elaters twist and spin to help scatter them.
Many liverworts (and some mosses) also produce a special reproductive structure called a gemma cup. Within each cup are several small disks of green tissue, called gemmae. When gemmae are splashed out of their cups by water, each one can sprout into a new individual, genetically identical to its “parent.”
The Hornworts
In outward appearance and size, hornworts greatly resemble the liverworts. In fact, they were long considered a type of horned liverwort. More recently, scientists have discovered that hornworts have chloroplasts and pigments that resemble those of green algae more than that of true plants. For this and other peculiarities, botanists place the hornworts in a class by themselves.
Hornworts derive their name from their distinctive sporophytes, which sprout like green horns from the flattened body of the main plant (the gametophyte generation). In all, there are more than 300 species, found on wet rocks and damp ground around the world. The male and female sex organs of hornworts (antheridia and archegonia) often occur on the same plant. They have no stalks, but lie sunken into the plant’s flat surface. The hornwort’s sperm therefore need only swim out of the plant’s antheridium and into a nearby archegonium to fertilize an egg.
The fertilized egg gives rise to the spore-producing structure that gives the hornwort its name. It grows as a tapered cylinder, or “horn,” sometimes attaining a height of up to 2 inches (5 centimeters). When mature, each spore case splits open into ribbonlike halves from the top down and releases millions of spores.