This sheltered little nook, surrounded by rocks, is in full shade most of the time. During one of the rare times when the sun reaches this corner, the camera caught these sweet little liverworts in the process of covering the damp ground.
Liverworts are among the most primitive land plants on the planet. They first grew on land some 472 million years ago, during the mid-Ordovician, before the first animals crawled out of the water. The Wikipedia article has lots of fascinating details.
Little more than flat sheets of cells, these ground-hugging, bright green shapes are not actually leaves, although they may look like them. Each “leaf” is called a thallus, a word for a more or less undifferentiated plant body. Together, they are called thalli.
In the above close-up, you might be able to make out the fine, stippled texture of the surface of the thalli. The tiny raised dots are the reproductive structures. When there is liquid water, the plants release motile male cells, which swim through the water to the waiting female ova. The liverworts also reproduce prolifically by vegetative division and growth, rapidly covering the ground in good conditions.
A nearby rock has some patches of nice green moss and some light gray lichen thalli. Both are also quite primitive. The moss is more advanced than the liverwort, bearing leaflike bracts on tiny stems, while the lichen is a symbiotic combination of algae and fungus.
Look around in sheltered, moist undisturbed parts of your garden. Are there any liverworts? They are a sign of a healthy micro-ecosystem.