Lichens and Mosses
These primitive plants serve important functions in nature
Lichens and mosses played important roles in plant evolution and continue to be essential to the processes of breaking down rock and wood and building soil.
These primitive plants preceded and paved the way for the development of the so-called higher plants, the flowering species that have vascular roots (meaning that they contain hollow vessels that can transport water and nutrients more efficiently, but are longer and therefore require deep soil. They also have the capacity to penetrate to deep layers of soil to seek water and minerals, and therefore they have greater potential to survive in a variety of habitats. At the time lichens and mosses evolved, scientists believe that the earth’s surface was covered with oceans and rock. Certain algae, aquatic plants from the oceans, united with fungi to form lichens, while other species evolved into mosses, both groups migrating from the ocean to live on rock.
All lichens are composed of two separate plants, an alga and a fungus, intertwined at the cellular level, giving the lichen great flexibility. It colonizes both dead and living trees, as well as rocks. The alga, which contains chlorophyll, manufactures food in the presence of water and sunlight. The fungus absorbs food from the alga, provides structure, and contains acids that break down rock and wood, providing minerals to nourish the lichen and leaving behind microscopic fragments of matter that, over centuries, accumulate to form dirt.
Mosses have rootlike appendages called rhizoids, which anchor the plant in shallow soil and diffuse water and minerals through their cells, less efficiently than real roots do, but well enough to sustain a fairly complex plant. The mosses evolved to use the small amounts of soil created by the lichens, and then helped build up a thicker layer by catching windblown dirt in their interlocking system of rootlets and upraised stems. The accumulation of soil made possible the evolution of higher plants with real roots. These dirtbuilding processes continue today, right under our noses.
Both mosses and lichens can tolerate extremes of moisture and dryness. They love dampness, and it takes a long period of submersion in water to kill them. Under dry conditions, they go dormant. Mosses will turn brown, withdrawing their life force into the rootlets and springing back to green as soon as moisture returns. Under optimal conditions, they produce spores that enable them to reproduce and start new colonies. They are also able to grow appendages that are capable of being broken off and living independently.
