Winter-Thriving Woodland Mosses
Lowly, primitive mosses very often remain unnoticed, although they are everywhere around us in the countryside. They are most obvious in the winter months, when they are often the only green colour in a woodland shorn of summer vegetation.
Many mosses grow actively in the winter when the climate is cool and the moisture abundant, and they are at their most luxuriant after the winter’s growth. In the spring they produce their spore capsules, and then gradually wither and dry as the summer progresses, repeating the cycle when damp conditions return.
A dry stone wall or a tree may appear to have an unchanging population of mosses. However, an interesting study of the rare moss Grimmia stirtoni, which has been known for eighty years at a site on a Scottish island, revealed that an apparently stable population of moss cushions was really a constantly changing one. The lifespan of the individual cushions was quite short-a matter of a few months only. This is probably true of a great many mosses, especially those living in harsh situations such as stone walls where new cushions appear each year.
Mosses are among the few plants still existing that retain some of the characteristics of the very earliest land plants. They have no ‘true’ leaves, stems or roots. The moss stem lacks the xylem vessels that are normally used to conduct food and water up a plant stem. Instead, mosses absorb water over their entire surface, and so every moss cell must be within reach of the growing surface to obtain its water supply.
Moss leaves are thin and flat and arranged spirally around the small flimsy stems. True leaves, such as those found on flowering plants, have stomata in their surface-these are pores bordered by special cells that can open and close according to the humidity, allowing air to enter the leaf. Moss leaves do not have these air holes, nor do they have roots, these being replaced by rhizoids – thin hair-like outgrowths that attach them to the ground.
Mosses also differ from flowering plants in reproducing by a system of alternation of generations. A moss plant carries both male and female organs and when the female part is fertilised, the second, or spore-bearing, generation is produced. The spore capsules dry and releasewhich, provided they land in suitable conditions, germinate to form a new moss plant.
As mosses need a constant water supply, they grow best in damp or wet places, although some species survive on stone walls or fences, provided the conditions in the spring are suitable for the production of spore capsules.
Some of the most spectacular mosses are seen in very wet places. Upland streams such as those found in Scotland and northern England often arise from springs; the water brings a constant supply of minerals and oxygen to the mosses and liverworts that grow on the ledges and. Philonotis fontanel forms a bright green carpet in such a situation, and it is often accompanied by the cushion-forming Dicranella palustris and many other mosses as well. On the banks of woodland streams species such as Polytrichum formosum, one of the hair mosses and Mnium punctatum, grow along with many other mosses and liverworts. The genus Polytrichum includes some of the largest of all British mosses and Polytrichum commune may reach up to 30cm (12in) in length in very wet conditions among long grasses.