Sedges growing amongst the paving in my garden
I have these beautiful, what I thought was grasses, springing up all over in my bathroom court-yard, but upon trying to identify them, found out that they are sedges. Grasses are characterised by 'nodes' along the stem - or jointed stems - as opposed to 'sedges' which have stems with no joints and which belong to another family.
As in grasses, the basic unit of the inflorescence in sedges is the spikelet. Within the family there is enormous variation in spikelet and inflorescence structure. The spikelet consists of one to several, tiny, male, female or bisexual flowers, each borne in the axil of a boat-shaped glume (tiny bract) which is coloured in shades of green or brown, red, occasionally white or bright yellow. Mostly petals are absent, but when present, they are concealed within the glume and consist of bristles or scale-like structures.
Distribution
The family comprises about 104 genera and more than 5 000 species world-wide, although estimates of numbers vary greatly due to differing taxonomic concepts of individual researchers and because modern sedge floras are available for only a few countries. The largest genus is Carex with about 2 000 species world-wide, followed by Cyperus with about 550 species. Sedges occur primarily in the tropics and subtropics, but may be locally dominant in some areas like the subarctic regions (tundra).
In rather arid southern Africa there are roughly 40 genera and 400 species. They are found throughout the region, in particular habitats. Some species e.g. in the genus Tetraria are endemic (occur nowhere else in the world) to the winter rainfall region of the Western Cape, South Africa.
Ecology
In southern Africa sedges are found mainly in wetlands (some are entirely aquatic) and along watercourses, but also occur in moist grasslands and along forest margins. Some genera (e.g. Tetraria) are common constituents of fynbos vegetation, which generally occurs on impoverished sandstone soils, whereas several species of other genera are pioneers on coastal dune sands.
It is not surprising that adaptations to particular habitats are many. Many species are deciduous, and survive the unfavourable season as rhizomes, corms or tubers. Several species grow in semi-arid areas, and are able to survive periods of drought due to succulent, water-storing leaf sheaths. In arid areas many species have overcome the problem of temporary moisture (such as in pans) by becoming annuals, completing their life cycles in a month or two. Species occurring in fire-regulated grasslands often have protectively thickened and hardened or fibrous leaf sheaths.
Sedges are generally wind pollinated, although some of the brightly coloured species are thought to be insect-pollinated. On several occasions I have observed thrips (tiny insects) in the flowers of Fuirena species, absolutely covered in pollen. It is almost certain that, as well as feeding on the pollen, they are also vectors. After disintegration of the spikelet the fruits are mostly dispersed by wind and water, and can also be carried long distances in mud on the feet of migrating birds. Some fruits have corky flotation tissue or further adaptations for water dispersal, some have elaiosomes (an oil-rich appendage on the seed) for ant dispersal, and others are equipped with tiny hooks and bristles for dispersal in the fur and feathers of mammals and birds.
Economic and cultural value
The chief importance of sedges lies in their forming a major natural constituent of wetlands and riverside vegetation, where their densely tangled rhizomes contribute to erosion control and water purification. The consequences once they are eradicated are unfortunately all too easily observed. While on the natural theme, the dense sedge beds that form in swampy regions provide food and shelter for birds, animals and other aquatic life-thus attracting ecotourism. In grasslands, terrestrial game birds (e.g. francolin) feed almost exclusively on the small corms of some Cyperus species.
Info from PlantzAfrica
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