After heavy rains we had in early April, many of my trees, and rocks, sported lichens and mosses, totally fascinating! And now, two months later, they are still there, but have turned pale grey. But understanding lichens is complex (to me at least!), so I've had to resort to an internet search to explain them.
Worldwide, over 20,000 species are known. A lichen is not a single organism. Rather, it is a symbiosis between different organisms - a fungus and an alga or cyanobacterium. Cyanobacteria are sometimes still referred to as 'blue-green algae', though they are quite distinct from the algae. The non-fungal partner contains chlorophyll and is called the photobiont. The fungal partner may be referred to as the mycobiont. While most lichen partnerships consist of one mycobiont and one photobiont, that's not universal for there are lichens with more than one photobiont partner. When looked at microscopically, the fungal partner is seen to be composed of filamentous cells and each such filament is called a hypha. These hyphae grow by extension and may branch but keep a constant diameter. Amongst the photobionts there are those that are also filamentous in structure while others are composed of chains or clusters of more-or-less globose cells.
Given that they contain chlorophyll, algae and cyanobacteria can manufacture carbohydrates with the help of light via the process of photosynthesis. By contrast, fungi do not make their own carbohydrates. Every fungus needs existing organic matter from which to obtain carbon. In a lichen some of the carbohydrate produced by the photobiont is of course used by the photobiont but some is 'harvested' by the mycobiont.
Lichens are classified with the fungi (being sometimes referred to as lichenized fungi) and can be found growing in almost all parts of the terrestrial world, from the ice-free polar areas to the tropics, from tropical rainforests to those desert areas free of mobile sand dunes.
Foliose lichens could be thought of as halfway between crustose (two dimensional) and fruticose (erect). Though obviously three dimensional, they grow in a more-or-less sheet-like form, but often with a lobed appearance. They are not attached by their entire lower surfaces to their substrates. Indeed, some foliose lichens are just centrally attached.
Another part of the same tree trunk, lower down, was covered in moss. Now Wikipedia says, "Mosses are small flowerless plants that typically grow in dense green clumps or mats, often in damp or shady locations. The individual plants are usually composed of simple, one-cell thick leaves, attached to a stem that may be branched or un-branched and has only a limited role in conducting water and nutrients."
The same thick mat of moss also formed on a rock on my patio, but both of them never quite got to the thick stage and have now all but disappeared. The moss life-cycle starts with a haploid spore that germinates to produce a protonema (pl. protonemata), which is either a mass of thread-like filaments or thalloid (flat and thallus-like). Massed moss protonemata typically look like a thin green felt, and may grow on damp soil, tree bark, rocks, concrete, or almost any other reasonably stable surface. This is a transitory stage in the life of a moss, but from the protonema grows the gametophore ("gamete-bearer") that is structurally differentiated into stems and leaves. A single mat of protonemata may develop several gametophore shoots, resulting in a clump of moss. But they do need a lot of damp to continue surviving.
Most mosses rely on the wind to disperse the spores, but It has recently been found that microarthropods, such as springtails and mites, can effect moss fertilization.
It seems that, given enough moisture, moss might still take over the world! lol!