4 April 2008
Moss
Posted by bryanc under: biology; nature; science .
It is common to associate moss with moist environments; in the temperate rain forests of western Washington and Alaska or covering trees and decaying stumps in the tropics. For one to cognitively link moss to moisture is not necessarily wrong because their entire method of reproduction relies on the presence of water. However, as illustrated in the picture above, mats of moss can grow in the relatively cold and dry climates of northeastern Washington. In fact, they are found in the hot deserts and the cold tundra; both of which are seriously lacking in moisture. How does this mat of moss survive and reproduce on something as barren as the surface of a chunk of limestone rock?
The ability for moss to survive in such marginal environments as the surface of a rock is a product of their extremely low maintenance. They are small and they are tough. The predominate generation of a moss has no vascular tissue like trees or grass and has leaves that are only one cell layer thick. Because moss lack strong water and nutrient transport they must grow low, sometimes in mats, giving them less structural mass to maintain with nutrients, moisture, temperature, and other concerns. The single cell layer of the leaf allows gas and water to be absorbed quickly and efficiently. Pore some water onto a mat of dried moss to see how quickly the plants spring to life.
The reproductive cycle of the moss is radically different from what may be intuited by a naive observer. The dominate life stage of a moss, the structure most of us are familiar with, is the gametophyte which is the stage where only one set of chromosomes is present in the organism. This is different than, say, a tree or a human. Both of which have two sets of chromosomes (the tree’s pollen and egg have one set just as our sperm and egg do). The male gametophyte produces sperm which must use water as a transport vessel to the female gametophyte. Fertilization occurs from the union of the egg and sperm and a young sporophyte (two chromosomes, one set from the male, one from the female) grows as a sort of parasite right out of the female gametophyte. The sporophyte matures and releases male and female spores that grow into male and female gametophytes. The cycle continues. [Note: the yellow in the picture above is the sporophytes and the brown/black is the gametophytes]
As stated above, water is required by the moss to transport the sperm to the egg. This is the exact reason why mosses grow best in moist areas. Because sperm is tiny, made of only a few cells, any minute bit of water is able to transport a bundle of them. A single drop of rain splashing onto a male gametophyte can engulf a group of sperm and transport them to multiple eggs. Thus one raindrop, timed and placed perfectly, can sustain a mat of moss for another generation.
