Marchantia polymorpha L. (Marchantiaceae).
Marchantia polymorpha is a morphologically variable liverwort that naturally grows in a wide diversity of damp environments, including gardens and greenhouses, where it can be an annoyance to tidy gardeners.
For much of the year Marchantia polymorpha grows flat along the surface of the ground, anchored there by a dense network of fibre-like cells called rhizoids.
Each spring these tiny plants produce small, aerial umbrella-like structures. There are two types of umbrellas; male umbrellas (antheridiophores) with a disc like top, and females (archaegoniophores) with a star-shaped top.
The antheridiophores produce masses of sperm that swim through the water, which coats the plant's surface, to nearby females, where they fertilize eggs located under the arms of the star-shaped archaegoniophores.
After fertilisation, a sac, called a sporangium, containing up to 200,000 spores -miniscule, seed-like propagules - develops. At maturity the sporangium bursts, releasing powdery spores which are carried by the wind and water. They eventually settle on the ground surface, germinate and develop into new male or female plants.
Spores have a very tough protective coat containing one of the hardest known substances - sporopollenin. The sporopollenin coating seals the moist internal contents of the spore and prevents them from drying out. The water-resistant walls of these spores are so tough that they can survive millions of years of burial in muddy sediments that over time turn to rock - the spores become microfossils.
In a search for evidence for the first land plants, Charles Wellman from the University of Sheffield, dissolved lumps of 470 million-year-old shale with a cocktail of strong acids. These acids completely dissolved the rock except for the hard, organic remains that were once trapped in the muddy sediments from which the rock formed.
Charles inspected the miniscule amounts of organic residues with a microscope and found that they contained the remains of ancient spores - 470 million-year-old spores. Detailed analysis of the structure of the spore walls, using electron microscopes, revealed that they were made of consecutive layers, like the concentric rings of a cut onion.
The only living plants with onion-like layers in their spore walls are liverworts. This discovery showed that some of the first plants to grow on land - or at least the oldest plants of which we know - had liverwort-like spores. This suggests, though does not prove, that the earliest land plants were probably related to the Marchantia polymorpha that causes so many headaches for gardeners today.
Wellman CH 2010. The invasion of the land by plants: when and where. New Phytologist 188, 306-309.